1. The registers haven't changed much between 6xx/7xx and later GPUs, so we've just been providing updated register header files for the newer hardware. We're going to try to get a new set of docs out for SI then go back to register header updates for a few more generations.
2. AFAIK preemption for DX is usually done in the OS scheduler with a bit of help from the driver, ie not purely in hardware.
3. Probably best to ask this on #radeon or #dri-devel
Note that the open source drivers run without X today -- Wayland and Android both run with just the kernel driver and mesa.