There's a fairly recent growth in the D community of a "better C" movement, that is, people using D for lowest-level systems development, device drivers, etc. This seems to have lit a fire under initiatives to reduce D's GC dependency, and its runtime dependency in general. I don't follow this too closely, but it seems they are still at the "hacks and experiments" stage in terms of real-world use (e.g., people writing custom runtimes that stub out the GC, etc.).

DConf (http://dconf.org/2017/schedule/) is in three weeks, and there are a number of related talks. Once the videos are out, you might get a better sense of the current "D as a better C" landscape.

Here is someone writing a kernel in D: https://github.com/Vild/PowerNex

Yes – there are a number of research and/or hobby projects that implement kernels, drivers, etc. in D.

Yes