Unfortunately I don’t have a perfect solution to this either – the closest thing I’ve found is an engine change which allowed materials to write to the velocity buffer directly. There’s a video tutorial on this, but I noticed that the video has become unlisted on youtube, so I don’t think posting it publicly is really appropriate. I’ll message it to you directly if you’re interested.
As far as the previous frame switch node, it seems like you’ve gotten farther than I ever managed. I’ve seen examples that appear to work, but only as long as some data is passed into the material as a parameter. I haven’t been able to make it do anything at all in a “self-contained” material even with “Accurate velocities from Vertex Deformation” turned on. Any chance you could share your node setup?
I’ve gotten something that “sort of” works by applying a rapidly oscillating WPO along motion vectors, but it’s still definitely not a great substitute.