I agree with Beckles. Given your choices Delta seems the way to go.

United is a partner of SQ, earning 70% in economy, all I can tell you is my M class this weekend got UA mileage, but some classes are probably excluded. 1000 miles isn't much to worry about abondoning, adn well now they wont expire, in the future you may have a use for them.

LH with the higher mileage requirements wipes out the miles you have with them, so changing to delta doesnt matter, you will get more for the 32K (or less with the 70% rule) with delta than LH. If you end up short on the miles, you can top up the delta miles using a partner, say for example MCI, get a MCI phone card for 5,000bonus miles plus miles for your calls...

I wouldnt change the SQ flight, I love SQ, and the 70% economy rule doesnt bother me, losing 30% versus flying some of the other carriers isnt much to give up for the better flights, service, etc. Now Air France... that you could change... but note that Air France miles do count towards Delta elite status, but you would have to match those AF miles with some more long trips to make use of that since you dont already have status with them...
I think continental is also with AF, but not SQ. YOu might consider not going for the free ticket but instead changing the miles into Hilton points and using them for free/discounted hotel stay which might make them more valuable then the ticket if you can find a cheap ticket for that US trip you want, and that way you get teh miles on that one too...
good luck.