After testing other clips and filters/effects, iIt seems the Avid FX filerwas the issue with this problem. I eventually gave up trying to add Timewarp to the clip, exported it out and back into Avid, and then applied Timewarp to the exported clip.
I don’t yet know why the Promote button is not on my Motion Effects Editor. I tested that out also on different clips, with no effects preciously applied to them, and it never once showed up in the MEE pop-up. I’m sure I’ll stumble on the solution someday!