Chappell had an agenda. He thought the Scots had been given far too much credit for originating tunes that were actually English. And he went way too far in the other direction. Not crediting Motherwell would fit in with that. What he says in that page makes it clear he did NOT get it from any written English source he could reference.

Collectors transpose tunes all the time, for many reasons. It's common now for collectors to state the key they found the tune in - Bartok and Grainger did that - but it wasn't in Chappell's time.

I'll ask Karen Macaulay.