Hi Trish,
Everything to do with David Donaldson has me confused.

I don't think David belonged to William. If you read what it says on his Marriage and Death cert, it sounds like he doesn't;
MARRIAGE: Parents listed as Jeanie Wyllie (now married to William Roberts, Masons Labourer)
DEATH CERT: Parents Jeanie Wyllie Domestic Servant afterwards married to 1st William Roberts Masons Labourer, 2nd James Smith Coal Miner (deceased)
David went onto have 10 children after he got married and one thing that has interested me is that 8 of them have middle name of "Donaldson" and only one with "Roberts". I'm guessing, although I can not confirm his biological father may be a Donaldson?
The change of ages are confusing aren't they. David was illiterate like his mother, so I'm guess that is why they changed so much.
I do not have an 1881 census showing William, I don't know where he was then.
Hanan