Why be puzzled at a gap with print? Only a tiny fraction of what was printed has survived, moreso the further back we go.

George's missus, always possible, but not much point in the conjecture with no corroborating evidence. As I say, we can't assume anything without some evidence. I'd be more inclined to think 'scarlet' was a pun on 'Reading'. The 'scarlet' is specifically the name given to the town so I don't see how that could be describing Mrs Villiers.

What would be useful is a contemporary reference to 'Reading' as 'scarlet' in a piece of literature.

Well, Martinmas is neither more Scottish nor more ancient.

Even if what was sung in 1666 was the Ramsay version the Scarlet/Reading could still be older. Without further evidence we simply don't know. All we can say with any certainty is that one or other version of the ballad has existed in print and oral tradition for about 350 years.