Maybe I was led astray a long time ago, but I was told to ALWAYS ingest clips that are over-cranked (in my case, 1080P, 60fps, or 120fps) into a separate project that matches the source clip frame-rate, then drag those clips into the main project (23,98fps) in order to be able to use time warp on said clips.
Today, on v2018.7.1, I transcoded the 60 and 120fps clips directly into the main, 23.98fps project at their NATIVE frame-rates, and time warp worked perfectly.
1. Has this always been the case and I've been creating a whole bunch of unnecessary work in the past, or is this something new?
2. When I uprez to UHD, can I assume all the time warps will uprez properly?
There was a version when source side playback was added combined that things got easier and one can mostly stay in the same project now. I have always hated having to create a project per frame rate, projects per resolution, etc. One day (year) Avid may finally free the timeline resolution from the project type...
With AMA linked media at a different rate, you can go to its source settings and set playback rate to original or project. In your example, setting it to project would play back frame for frame slomo (or faster depending on direction). It is always a frame for frame and audio will stay in sync with it, but not be pitch corrected. See my blog and spreadsheet calculator for pitch correction depending on speed changes: http://24p.com/wordpress/?p=195
You can still apply motion effect to it, but it is a quick way to get "frame for frame" slomo on all sources out of the gate.