If your question is not answered here you can ask in one of the communities.
You can check the wiki for more information about compatibility and known problems with specifc games.
The GamePad (DRC) screen can be displayed by holding the Tab key or by mapping it to a controller button. You can also toggle between the screens by pressing Ctrl + Tab. Additionally, the GamePad view can be displayed in a second window.
Cemu supports most online features. In order to connect to official servers some system files must be dumped from a Wii U. You can find a guide here.
At this point Cemu can already be considered highly optimized and possible future optimizations will only arrive in small and incremental steps. Keep in mind that emulation always comes with overhead and requires a higher performance system than the emulated system. For Cemu, the single core performance of your CPU (and by extension RAM speed) is the most important factor for fullspeed emulation.
No, Cemu frequently exceeds the usage of more than 4GB of RAM and therefore cannot run on 32bit systems.