If your questions cannot be answered here you can ask at one of the communities.
The GamePad (DRC), Pro Controller & Classic Controller can be emulated, which can be mapped to either your keyboard or any USB controller. Cemu currently only supports the DirectInput API. However, XInput support is planned in a future update.
The GamePad (DRC) screen is rendered into the main window of Cemu while holding the TAB key on your keyboard. You can also toggle between the screens by pressing CTRL + TAB.
Yes, mostly. But be warned, some sound effects and filters are not yet supported and there can be sudden loud noises in certain games.
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 are thousands of other little things that need to be done before work on ports can be started.
Even though Cemu is already partly optimized, emulation always requires a relatively high performance compared to the emulated system. Especially the single core performance of your CPU is the most decisive factor for your Cemu experience. Further optimizations can be expected with each new update of Cemu, but huge performance increases probably won't happen anymore.
Cemu uses mostly only two threads, one for the CPU and the other one for the GPU emulation, and therefore won't benefit from a CPU with many cores. This design was chosen due to the architecture of the Wii U. The console has three cores, but most games only utilize one. In the past, we already experimented with three CPU threads, however, the overhead and the synchronization necessity caused problems, while not improving the overall performance at all.
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 not perfect yet and it's not unusual that games and applications just don't work. You can check the Compatibility Page for more information about problems with certain games.