Information is in previous discussions; no need to start a new thread so that people can repeat the same information (or misinformation in "Foolstroupe's" case). Search engine is at the top of every page here.

There may well have been confusion at some point between "full" and "fool", but we don't know what the original was; though, since the song was first published in England c.1588 (the first text surviving is Ravenscroft,1609) while the first known Scottish example didn't appear until 1701 (and that was just a tune without words published by Henry Playford) it might seem reasonable to suppose that "fool" is the original. So far as I remember, the first published Scottish text including the line "Wha's fou, wha's fou?" didn't appear until 1827 (Kinloch, Ballad Book).

Rather a big claim to say that "the English stuffed it up" when the first known Scottish text we have is more than two centuries later than the earliest surviving English one. Unless, of course, "Foolstroupe" is privy to information hitherto unknown to scholars; in which case I think we should be told.