1) I haven't heard about problems other than I can tell you the 14 and new transmission is way better than previous.
2) I have the 9 speaker system non-Beats with sub in a 2015 RT. Plenty loud. Not quite as well defined as some high end systems from other makes, but very good. Plenty of Bass for rock and hip hop. A little lacking for the detail in classical.
3) I don't think RTs had the option to have a roof rack in 14-16. Have never seen such a thing myself and I looked for it.
4) Be aware that in some models the front passenger seat folds down. That is really handy for carrying long lumber or ladders, especially if you don't have a roof rack. But models with a full power recline passenger seat can't do that. My 2012 crew passenger seat folded, my 2015 RT does not. Also be aware that the stock (in most vehicles) Goodyear Fortera tires are good for about 40,000 miles. Beyond that my experience is that they will not perform well in snow even if the tread looks decent.