Once you have cleaned up some of the "niggly details", then you can see more clearly what is happening with the structure of the senses and subentries. Now is the time to try to get those in the right places.
Senses: A common problem with MDF files is that records without \sn in single-sense entries have all of their sense-level fields showing as an error (e.g., \ge, \de, \re). This is because Solid only allows one parent per cluster of markers, whereas the FLEx import is able to infer it.
You could go ahead and let one or more of these markers "infer \sn" in case of an error. Eventually you may do this. But you may not start out with that.
If you want to learn anything about where the real \sn markers occur, do your investigating before you set up the "infer" rule. Once you are satisfied that you understand how they are used (e.g., in relation to \ps or \se or other markers), then go ahead and set up the "infer" rule.
If you are setting things up to infer \sn above sense-level markers, it will be most helpful if you only have to do it for one of them. See if there is a certain marker that always occurs first, and see how far you get with setting up the infer rule for only that one. (But if the marker that occurs first is allowed to occur more than once in the sense, that won't work.)
If the database has subentries, determine if they ever occur between senses. If so, you may have "subentries of senses", and this will be a more complicated import. [TO DO: See section ??? for more info]
Determine which fields inside a subentry need a different marker from the same fields at the entry level. There are two reasons this might be needed:
There are some fields for which this matters for the FLEx import. [TO DO: Describe how to tell, or point to a document that tells.]
If you will be promoting subentries out of senses, then that script needs to know how to tell it has found the end of the subentry. If the marker that indicates you have left the subentry, is the same as a marker that can occur inside the subentry, then one or the other needs to be different. (For instance, if you have \sn both in the mai entry and in the subentry, you may need to make all the ones inside the subentry into \snSE. This has to be done by hand.)
[TO DO: Add some tips about how to tell.]