1. No idea, it should just work. Latest LE11 nightlies have fixes for some situations where blank screens might be seen. Again, I suggest you try the latest nightly on a clean SD card with no attempts to pre-fix problems.
2. RPi websites should have authoritative documentation. The main point of the switch to the Linux kernel DRM framework is that most of the old config.txt hacks available in the older downstream vendor kernel should not be needed now. Users assume reduced documentation on config.txt means something is missing, but that's not correct. You can capture the EDID data from HDMI and force detection of the TV; run 'getedid create' with a working HDMI connection.
3. Nope. Never seen it reported (never seen it myself). You can force output modes (as previously hinted) but you cannot force colourspace.
Current Kodi skins are 1080p resolution so either Kodi or the TV has to do upscaling to 4K and generally keeping Kodi on 1080p and allowing the TV to do the upscale results in lower load on the RPi (so faster RPi) and equivalent or better results on-screen as the TV has dedicating scaling hardware. So far that that guidance seem to be true for the majority but I guess the results are ultimately subjective.