1. DNA looks like a nice phone but just know that it is also a big phone. I would make sure to look at it in person. Also look at the Samsung Galaxy S3.
2. Yes you can if you root your phone and install a ROM on it (if a 4.2 ROM exist). A lot of people mess up their phones because they attempt to root and install a ROM without doing proper research. This forum is littered with post from users who attempt to root their phones and got in over their heads. I would visit xda-developers.com and read up on it. Also make sure you read up on how to unroot a phone and it may come in handy.
3. No, you can not just take what Google releases and install it on your phone. Basically a developer will take that code and modify it to work on your phone. Once again I would visit Android, Windows Phone, and Windows Mobile Development News, Information, and Howtos - XDA Developers
, particularly the HTC DNA section if that's the phone you choose.
4. Your guess is as good as mine.
Past Devices: Google Nexus One (sold), Samsung Galaxy S2 (sold), B&N Nook (returned), Samsung Galaxy Tab 7 (sold), Samsung Galaxy Tab 10.1 (On the Shelf)
Current Devices: Samsung Galaxy S3 (AOKP), Nexus 7 (Stock), Chromecast