1. Yes, the I35 is the later, 3.5 liter engined, version of the I30 (3.0 liter engine).
2. If you have some knowledge of audio electronics you can do it. Check Maxima.org. I think someone on that site has some information about it. I think you may need an adapter to hook up. By the way the Maxima, I30 and I35 are mechanically identical. Almost all parts are interchangeable between these cars.
3. Bubbling paint is not covered by any warranty unless specifically identified as a covered item. Typically that type of problem is covered by the basic warranty which, I'm guessing has expired. The corrosion/perforation warranty usually covers rust through, a hole caused by rust.
4. There is an ATF flush write up on the Maxima.org site.