Ah, that's all much better, and quite clear. It's always best to include everything you know already when you start a topic!
I think you are right about the unnamed child, and a few small discrepancies in age in later censuses are not unusual. However he was born on 15 December 1855 (the date in the IGI at FS is the date of birth, not the date of registration) so he was almost certainly married before he had had his birthday in 1888, in which case 32 is exactly correct. Also, the census was taken around the end of March or beginning of April, so on census day 1891 he would have been 35 years old, because his most recent birthday was in 1890. So if he said he was 34 in the 1891 census, that is only one year out.
There are a few things I would suggest. First, an 1855 birth (or any other) certificate contains lots of information that was never collected in any earlier or later year, so I would recommend getting a copy of it if you don't already have one. It will give you information that should lead to identifying Edward Johnstone and hence what became of him, your biological ancestor.
Second, have you found John in the 1861, 1871 or 1881 censuses? If he was living with Jane/Jean, this should confirm the identification with the unnamed child.
You say that Margaret's birth was registered as illegitimate. How, exactly, was her mother's name listed on her birth certificate?
If you have failed to find John's death in Scotland, I would certainly recommend looking in England, though this isn't going to be easy as it is such a common name, but at least you know that he was about 70 when Betsy died, which will cut down the number of candidates to consider. I can't see a likely candidate in Somerset, however.