Probably you'll have to give the New Delta your DL and NW numbers for them to combine. That's how the US Airways/America West FF program mergers worked.

They are keeping NW's excellent IT systems for the loyalty program from what I have read, and they would probably be able to figure out what accounts belong together by name and address matching in many cases. However I think that for security/privacy reasons it is going to be an opt-in where you have to provide them the numbers. If you don't, you'll end up with two separate Delta accounts.

No meaningful way to get your miles out of UA. It makes more sense to find some way to get a few thousand more miles into UA to get up to a reward level. If you're already at 15K, that's enough to upgrade a UA North American domestic flight, or a full-fare international flight. 20K is enough for an intra-Europe or I think an intra-Asia award. 25K gets you a domestic US/Canada award or some other regional equivalents.

edit: Welcome to FlyerTalk!