Jennyr - I think we're singing from the same hymnsheet here!
In response to Paul Davenport's post on 14th Feb (12.12pm), where he wondered about the propriety of re-combining (notionally) linked ballads, I said that my personal preference is for what I could best describe as a "single issue" ballad.
But I have little compunction about importing lines or whole verses from other versions if it helps me present a clearer, stronger story. In fact, I once set this as a task for adults studying ballads on a university outreach course : I sent them three versions of 'Lord Gregory', asked them to identify what they felt were the key story elements and then present their composite version in no more than 15 verses. Some of their efforts were really convincing, and one person in particular even combined half-lines as well as making up her own short phrases in a couple of verses. (I also did the homework, but failed - because I couldn't jettison an almost repeat verse near the very end, and therefore had 16 verses!)
I'm sure many, many singers do this, some more intentionally than others, and your description of your own approach, and that of the Davenports certainly resonates.
Mind you, at our ballad workshops, Gordeanna McCulloch and I are always reminding participants that any changes/imports have to be sympathetic -- so the king cannot send Patrick Spens "a quick text message/ and pinged it wi's ain hand"!