@Efriim Hello and welcome to the RetroPie forums.

I don't play N64 so I can't say what exactly is the best choice here, the recommendation is usually to use the standalone emulator for best performance and even so not all games are playable. In general, N64 emulation on a Pi is not ideal - other may provide better answers to this question.

The standalone pcsx_rearmed is no longer developed, use lr-pcsx_rearmed which is better supported.

It's not a bug, it's how the Sony drivers work and it's the OS/kernal that handles this. Register 'both' controllers (wired and wireless) and they should work depending on how you connect them.

There is a recent topic about Dreamcast games and their performance after the latest update - https://retropie.org.uk/forum/topic/21031. I don't use the emulator so much, so others may have a better response.

RetroPie uses whatever Raspbian's default is - so /tmp is mounted on the sdcard and not using the RAM (as most Linux PC distributions do). I assume it's because of the limited RAM memory available on such systems. RetroPie does not have any preferences here, so you can use whatever solution you like best.

Every emulator will use the GPU - directly or indirectly - otherwise they wouldn't be able to output anything on the screen :). I don't see why it shouldn't be possible to run a music player in the background, there's no limitation imposed by RetroPie - remember that the Pi image is just a Rasbpian Lite installation, almost unchanged from upstream.

ShanWan Controller Woes