Unrelated to any of these, specifically, but somewhat in the same vein. Source Filmmaker lets you do some pretty crazy stuff with assets from the Source 1 engine. When you export, there is an option to also spit out a depth pass that works the same way as other 3d apps. However, with the announcement of Source 2, this is kind of pointless.
The first thing that comes to mind is that speed ramps over XML will always be tenuous. I think everyone has their own way of doing them and no one cares if their way plays nice with anyone else. If you could come up with a way to do some thing like "Import Pre Conformed EDL" in Resolve, that would be ideal. That way your speed ramps are baked into the footage and a mark up language insn't trying to recreate that.
This might be a little hack-y, and it will add 1 more step to editorial, but, maybe it'll work. Once picture is locked, have the editor export a 4444 out of FCP, then reimport it. Dupe your locked cut timeline, and put that clip on the timeline above everything else. The editor goes through and cuts up the single 4444 file (should be able to hit down arrow and ctrl-v and fly through his part). Delete the original media, leaving your cut up 4444 file, then export and edl. Bring your 4444 file into Ae along with that edl and theoretically, you'd have a fix. I know thats a little convoluted and may only work for someone working in a more contained environment, but hell, it might be worth a shot. Or it will spark an idea for someone else and they can more elegantly help solve your problem.
...and I do get spacebar previews on Quicktimes so not sure what the issue is as described above about lack of spacebar playback...
If you still render to Animation codec, you don't get preview. If you render to ProRes or other modern codecs, everything is fine. ProRes 4444 is a great codec, theres no reason (that I can think of, in my admittedly limited experience other than broadcast delivery) why one would still need to use animation codec.