[wrxtasy]

How's the deinterlacing PQ and motion display quality (both SD and HD) compares i.e. to Amlogic + LE or juts to a budget HTPC

The 6 Core 2.39GHz CPU package in the ATV 4K has enough ponies to do full motion 1080i h264 or mpeg2 YADIF2x software deinterlacing with MrMC, same quality as you would get with decent Intel Hardware or AML Hardware deinterlacing. They are all really good.
For SD mpeg2 TV you can even enable a bit of Kodi picture - Post Processing.

TV Channel switching with my external TvHeadend server using TvH Predictive Tuning is virtually instantaneous.

Same question regarding the PQ with upscaling and downscaling quality (SD->FHD, SD->4K, FHD->4K and 4K->FHD)

Likely not quite as good as decent Intel if it uses Lanczos3 optimised or better for SD > HD upscaling, but it's still pretty good. I've no complaints regarding PQ on the ATV 4K.

HDR->SDR conversion's color/brightness accuracy. 

No such thing as accuracy I believe, because different SDR TV's will output different color results and they will not look as good as the equivalent SDR only Bluray.

Here in Oz, 32GB ATV 4K's are $209 - a bit of a bargain for what you get packaged under the hood IMHO.