I don't see what's "hard to make sense of"?
I raised two points - 1) I don't think Welbeck will get regular first team football and 2) Fergie has form for selling players he seems to rate if he gets a decent offer. Whats complicated about that?
You also seem to be getting a bit carried away. Welbeck has done very well for Sunderland this year, no doubt - but can you really see him getting in to the side before Rooney (New £200k a week contract) Berbatov (top scorer, probably new contract) and Hernandez (great impact, potential star)?
He's had some very good games for Sunderland this year, but don't get carried away - he's a worker, he's quick and has decent movement.
He might become a very good player but he'll need to play regularly, week in week out, and I can't see it happening at United.
I've no idea what Fergie thinks, nor whether Welbeck will want to come back and fight for a place - fair play if he does. I'm simply saying that if Sunderland came in with a (for example) £10 million bid then no doubt United, with their own transfer targets and other players who can play in Welbeck's position might be tempted to sell.