1. I don't know too much about the D2. I read the D2 and DX are similar in performance. Hopefully someone else can elaborate.
2. The DX has only been out for a few weeks. It will get 2.2 and will be able to run flash. I expect this phone to get good support for the next year and a half.. at least.
3. People have already rooted the DX. Loading custom ROMs is a different story though because of the bootlaoder. But with root, tethering is available as well as many other good options.
But just saw a tweet from birdman and they fixed the hole in 2.2 for the root. So it is back to square 1.