There is one possibility of what happened to John Munro born in 1767, eldest son of Alexander Munro and his possible first wife Jane or Janet Munro in the parish of Resolis. There are six children who were baptised in the parish of Farr, Sutherland to parents John Munro and Betty Duncan and the names of the first four children are remarkably similar to the Munro family of Armadale which is also in the parish of Farr, Sutherland. They were James Munro baptised in 1801, Donald Munro baptised in 1804, Alexander Munro baptised in 1806, John Mackay Scobie Munro baptised in 1809, Cathrine Munro baptised in 1812, and Robert Munro baptised in 1816. Five of these parish baptism records show that the parents were living at the farmstead of Grubeg which was also known as Grumbeg. The only one that isn't, for Donald Munro in 1804, shows that the parents were living at Grubmore which was also known as Grumore and is not far from the location of the other five. So far I have had no way of linking this Munro family to the Munros of Armadale even though they have corresponding christian names. If the father was John Munro, born in 1767 in the parish of Resolis, eldest son of Alexander Munro and his possible first wife Jane or Janet Munro, he would have been aged 34 and 49 when the first and last children were baptised.
A explained on page Possible origins of Barbara Mackay of Armadale, who was the possible third wife of Alexander Munro of Armadale, she may have had a sister named Janet Mackay who was married to William Gordon and who had five children baptised in Grumbeg between 1809 and 1817 before moving to the village of Armadale.
Betty Duncan, as Betty Munro, is found on the 1841 census as a widow in the parish of Loth, Sutherland, aged 66 with daughter Catharine aged 28 as the only other family member. Catharine's age shows she was born in about 1813 which is just one year out from the 1812 baptism mentioned above. Betty Munro's age suggests that she was born in around 1775 which would make her about eight years younger than John Munro who was born in 1767. The Robert Munro baptised in 1816 could have been named in the same way that John's brother Hugh Munro, born in 1768, named his second son Robert in 1791 as explained on page Possible parish and estate records (part 1).
If John Munro was living under the wing of Janet Mackay who was sister of Barbara, then he could have named his eldest son James in 1801 after Janet and Barbara's father, like how Barbara named her eldest son James Munro in 1796. If this John Munro is the same person born in Resolis in 1767 then he could have named his second son Donald in 1804 after the Donald Simpson who Alexander Munro of Armadale possibly named his son Donald after in 1802. Likewise, Cathrine born in 1812 could have been named after Donald Simpson's wife, Katherine Munro in Resolis.
If this theory is correct then when Betty Duncan had her children with John Munro in Grubeg from 1801-1816 she would have been aged 26 to 41 which might seem late. However, as explained on page Possible origins of Barbara Mackay of Armadale, the above mentioned Janet Mackay, wife of William Gordon, who was also in Grubeg (Grumbeg), had children at an even later age and this might have been because William Gordon had been serving in the army where he could not have children, including alongside Alexander Munro of Armadale. If John Munro had been in the army then the same could apply, especially as he named one of his children John Mackay Scobie Munro in 1809 who appears to have been named after the noted army officer Captain Mackay John Scobie of Keoldale (H.E.I.C Honourable East India Company). The Captain was tacksman of Melness in the parish of Tongue but removed to Keoldale in the parish of Durness in 1815 where he died in 1818.