If your questions cannot be answered here you can ask at one of the communities.
Currently only the GamePad (DRC) can be emulated. You can map it to either your keyboard or any USB controller.
Yes, partially. But be warned, many sound effects and filters are not yet supported and there can be sudden loud noises.
Multi-platform support is planned and so far many development decisions have been in favor of supporting multiple platforms. (E.g. choosing OpenGL over DirectX for rendering) But currently there thousands of other little things that need to be done before work on ports can start.
Since Cemu is in such an early state it is more sensible to focus on accuracy and then, once that's nailed down, start work on optimizations.
Despite what you may have heard, DX12 will not magically increase speed. It does perform better in some situations but none of those apply to Cemu. Additionally, performance isn't really a concern since OpenGL can be very fast if properly used. Featurewise, modern OpenGL and DX12 are equal, so ultimately there is not really any reason to support DX12.
No, while it would be technically possible to support x86 it's just not worth the time. Due to some technical restrictions a 32bit version of Cemu would run drastically slower making it almost useless.
Cemu is in early stages of development and it's not unusual that games and applications just don't work.