The following reply was made in a discussion of Israel's 430 years in Egypt:
1) The dwelling of "the sons of Israel" ...
Exo 12:40 And the dwelling of the sons of Israel which they have dwelt in Egypt is four hundred and thirty years;
Exo 12:41 and it cometh to pass, at the end of four hundred and thirty years--yea, it cometh to pass in this self-same day--all the hosts of Jehovah have gone out from the land of Egypt.
We know God renamed Jacob Israel (Gen 32:28). So the name "Israel" in Exodus 12:40 can certainly identify with "Jacob". We also know that "the sons of Israel" include both Jacob's direct sons as well as his descendants. Now when did "the sons of Israel" first come into existence? Were "the sons of Israel" in existence when Abraham came into Canaan? I don't see how that's possible, given that neither Isaac nor Jacob (Israel) had been born yet. No, the earliest point one could possibly find for the start of "the sons of Israel" would be when Reuben was born, which was at the very most 70 years before Jacob (Israel) came into Egypt. So the starting point for the dwelling of the sons of Israel could not have begun any sooner than 70 years before Jacob came to Egypt since before that time there were no sons of Israel! Now the only way around this literal fact is to do a little arm waving and say, well, the 430 years didn't really start with the physical birth of the first sons of Israel, but started when Abraham came into Canaan, even though Abraham is never called a son of Israel in the Bible, or referred to as Israel (remember that Abraham had been dead for at least 20 years before Jacob was renamed Israel). In any case, for this conclusion to be correct, that the 430 years began with Abraham, Exodus 12:40 would need to say "the dwelling of the son of Terah", or at a minimum "the dwelling of the sons of Abram" (but even this second option is impossible given that Abram's son Isaac wasn't born until 25 years after Abram entered Canaan, so the start of the "sons of Abraham" couldn't be coincident with his entrance into Canaan either).
2) While the misreading and misinterpretation of "the sons of Israel" by the short sojourn folks is troubling, an even bigger blunder is their idea that the 430 year sojourn of Exodus 12:40 includes time spent in both Canaan and Egypt since this completely disagrees with Genesis 15:13-14 which clearly indicates Abram's seed would be a stranger in "a land" not in two "lands". In the Bible, singular and plural words are important, and you cannot introduce a theory that the sons of Israel were strangers in two lands when Genesis 15:13 clearly shows one land is in view in regard to the 400 years. Yes, Abraham and Isaac and Jacob were strangers and/or sojourners in Canaan as well as Egypt, however Genesis 15:13 is written so precisely that we know their time as a stranger/sojourner in Canaan is not to be reckoned to the 400 years. If this wasn't true, then the Bible would certainly use a plural form of land instead of singular. A similar argument stems from Genesis 15:14 in regard to "that nation" (another singular word), where that nation refers to the one that Abram's seed would serve and whom God would judge. Here again this verse does not speak of "those nations" but of "that nation", so the view that "that nation" could refer to Canaan and Egypt doesn't make any grammatical sense. Also notice in Genesis 15:14 that "that nation" would be judged, and afterward Abram's seed would come out with great substance. Exodus clearly shows God brought great judgment against Egypt before Abram's seed came out, but there is no indication that God brought any judgment against Canaan before Abram's seed came out of Egypt. So here again is another important detail ignored by the short sojourn view.
3) A third issue with the short sojourn view is that the 430 years began with Abraham entering Canaan, and that (according to their interpretation of Galatians 3:17) coincided with the giving of the covenant to Abram. Yet if one carefully reads Genesis 12-15, it is obvious that God didn't make a covenant with Abram upon his entrance into Canaan. In fact, Abram went to Egypt for a time before such event occurred. There are other activities with Lot and the battle of Siddim that also occurred before God makes a covenant with Abraham in Genesis 15. So in fact some time, most likely something on the order of 10 years passes (Gen 16:3), before the covenant of Genesis 15 is made. But the short sojourn view ignores these things and attributes zero time span to these events, so that they begin 430 years on the day Abram entered Canaan.
4) A fourth issue with the short sojourn view is that the 400 years of Genesis 15:13-16 would need to begin 30 years after Abraham entered Canaan. But the Bible records no event occurring when Abraham is 105 years old that would indicate the beginning of affliction by Canaanites, or that Abraham was in bondage to them.
OK, now to your questions:
1) why not begin 430 years when Abraham went to Egypt in Genesis 12?
> first, Abraham wasn't a son of Israel (Jacob), so he couldn't be referred to by the phrase "the sons of Israel" of Exodus 12:40, and thus the 430 years couldn't begin with him. second, we know Abraham returned from Egypt to Canaan, and so if the 430 years began with him going to Egypt in Genesis 12, then you still have the problem of sojourns in two lands (Canaan and Egypt) and bondage to these same two nations, in disagreement with the singular land and nation of Genesis 15:13-14. third, there is no indication that Abraham was in bondage to the Canaanites, nor that he was afflicted by them. Even the relationship between Abraham and Abimelech was mostly peaceful, and Abimelech gave Abraham many gifts, hardly the behavior of an oppressor.
2) why not begin 430 years when Joseph went into Egypt at 17 years old?
> as far as I can tell, this is your first reasonable question, which if true would result in a 22 year error in the calendar. so the question is, could the 430 years that "the sons of Israel" were in Egypt begin when "one son of Israel" (i.e., Joseph) went to Egypt while the rest of Jacob's sons were still in Canaan? Hopefully you can see the discrepancy with your proposition now that it is laid out more clearly, that is, "one son of Israel" being in Egypt is not the same as "the sons of Israel" being in Egypt. Here again the detail of singular vs plural is important and can't be ignored. of course one could argue back that Joseph had sons during the years of plenty, which were 2-9 years before Jacob came to Egypt, and thus, they could qualify as "the sons of Israel". However a better way to consider the matter is to look for what the Bible says as far as "the sons of Israel" going into Egypt rather than trying to speculate about it. A verse that indicates this exactly is,
Gen 46:5 And Jacob rose up from Beersheba: and the sons of Israel carried Jacob their father, and their little ones, and their wives, in the wagons which Pharaoh had sent to carry him.
Gen 46:6 And they took their cattle, and their goods, which they had gotten in the land of Canaan, and came into Egypt, Jacob, and all his seed with him:
Here we find the exact phrase of Exodus 12:40 used in Genesis 46:5, providing a sure indication of when "the sons of Israel" entered Egypt and the 430 year clock began. Nowhere does the Bible say or teach that Joseph entering Egypt corresponds to "the sons of Israel" entering Egypt, nor is the phrase used in regard to Joseph and his sons. This is further supported by the verses just before the one you quoted (Exodus 1:5),
Exo 1:1 Now these are the names of the children [sons] of Israel, which came into Egypt; every man and his household came with Jacob.
Exo 1:2 Reuben, Simeon, Levi, and Judah,
Exo 1:3 Issachar, Zebulun, and Benjamin,
Exo 1:4 Dan, and Naphtali, Gad, and Asher.
Exo 1:5 And all the souls that came out of the loins of Jacob were seventy souls: for Joseph was in Egypt already.
Verse 1 actually tells us who "the sons of Israel" are, naming them in verses 2-4, so that we can avoid speculation and know for sure who this phrase refers to in the context of Israel's Egyptian sojourn. Also, consider again Genesis 46,
Gen 46:5 And Jacob rose up from Beersheba: and the sons of Israel carried Jacob their father, and their little ones, and their wives, in the wagons which Pharaoh had sent to carry him.
Gen 46:6 And they took their cattle, and their goods, which they had gotten in the land of Canaan, and came into Egypt, Jacob, and all his seed with him:
Notice that verse 6 uses the phrase "and all his seed with him". The same word "seed", used in Genesis 15:13 in the phrase "Abram's seed", is used here, which again encourages us that it was Jacob and all his sons and descendants who are in view. The interesting point learned from this is that when the 430 years began, not a single seed (descendant) of Abraham in the line of Jacob remained in Canaan, all went down into Egypt, so not one who was still alive was missing. All these things point to Jacob entering Egypt as the start of the 430 years.
3) where is Jacob mentioned in Exodus 12:40?
> I already answered this above, that Jacob was renamed Israel, but will provide the Biblical support here:
Gen 32:27 And he said unto him, What is thy name? And he said, Jacob.
Gen 32:28 And he said, Thy name shall be called no more Jacob, but Israel: for as a prince hast thou power with God and with men, and hast prevailed.
4) How do you reconcile the fact that God begins the 430 years from Abraham to the law?
> Galatians 3:17 does not support the question you ask,
Gal 3:17 And this I say, that the covenant, that was confirmed before of God in Christ, the law, which was four hundred and thirty years after, cannot disannul, that it should make the promise of none effect.
this verse mentions the covenant being confirmed before of God in Christ, not the covenant being confirmed before of God to Abraham. this is a case where you have to be careful to read the verse without impressing something upon it that it doesn't say. additionally, you have confused "confirming" a covenant with "making" a covenant. it might seem a subtle difference but in reality it is a very big oversight. And it's worth mentioning again that the Bible does distinguish making a covenant with Abraham and confirming it was Jacob,
Psa 105:9 Which covenant he made [cut] with Abraham, and his oath unto Isaac;
Psa 105:10 And confirmed the same unto Jacob for a law, and to Israel for an everlasting covenant:
5) And even if one was to somehow say that "Jacob" is in view in Galatians 3, God confirmed the covenant with Jacob when he was still in Canaan - Genesis 28:13-15, Genesis 35:10-12
> not sure the point here since the confirmation of the covenant normally asserted for this are the statements to Jacob in Genesis 46:1-4.
Gen 46:1 And Israel took his journey with all that he had, and came to Beersheba, and offered sacrifices unto the God of his father Isaac.
Gen 46:2 And God spake unto Israel in the visions of the night, and said, Jacob, Jacob. And he said, Here am I.
Gen 46:3 And he said, I am God, the God of thy father: fear not to go down into Egypt; for I will there make of thee a great nation:
Gen 46:4 I will go down with thee into Egypt; and I will also surely bring thee up again: and Joseph shall put his hand upon thine eyes.
Which occurred while Jacob was on his journey to Egypt, thus these things are coincident with Jacob entering Egypt. It is also helpful to recall the historical situation here, because Jacob had been back in Canaan (since his sojourn in Haran) for many years, and was now quite old. So Jacob had enjoyed the land of Canaan, the one promised to Abraham and Isaac, as well as himself. But suddenly he was being told he must leave this promised land for Egypt. Surely Jacob would have wondered why! Was God annulling his promise for some reason? Was God changing the promise? I mean, why does he have to leave the promised land if it has been given to him? It was in this midst of this that God comes to Jacob and encourages him, and confirms to him that the covenant which God made with Abraham was still in force, it would surely come to pass (in fact, God confirms it to Jacob as a law, thus guaranteeing it). And with these words one can imagine Jacob's mind was eased significantly. So the confirmation of the covenant to Jacob at this point was no small event.