I haven't any access to Percy's manuscripts, but Child may well have had access via a correspondent as he had people like Furnivall checking on such things for him.

As you must be aware Child did make mistakes occasionally and he did take a FEW liberties in altering words to either standardise or make better sense which in his day even was standard editorial practice. (See David Atkinson's new online book). With any mammoth gathering and editing like this there are bound to be some inconsistencies.

I would expect 'dery' in a ms to be corrected to 'derry' as this is a very common refrain word in printed balladry, but if Child or anyone wanted to retain what was in an original copy this would be nowadays desirable.

If Parsons was just taking down what was actually sung at the time (1770) it may well be that the singer accidentally missed out the second verse and if Parsons wasn't familiar with the ballad then he didn't realise until Percy pointed the error out to him which would explain the extra verse being sent later.

As for Child's repeating the error it could well be that the same process happened between his transcriber and his receipt of the variants, i.e., his transcriber sent Child the 1770 version.

In many cases Child was at the mercy of transcribers in Scotland and England so any errors are not necessarily his.