I was in your position 6 months ago, asking myself much the same questions, here's my take...
1. Code now is really APA102 or 2812b, the only definite advantage of the latter is they can be much cheaper. In theory, the APA102s are simpler to set up and going by the posts on here more problems are had with the 2812b although a lot of those seem to be down to user error and poor wiring. If you buy APA102s direct from China you can get them for a reasonable price, which is what I went for.
2. Most of the guides recommending 30/m were written when 60/m were still very expensive but as to which is best depends on your set up. If your tv is very close to the wall 60/m will give you smoother colour as they create more overlap, as you move away the light can spread more so the advantage lessens.
3. The LEDs will only draw as much power as they need so that's not a problem, it when people try to get away with too small a supply that you get problems.