I'm sorry to hear about these problems, that's really a terrible and frustrating situation...
Sincerely I would expect a better compatibility with a conventional frame in terms of tubing like the UFO, for the $AFT. Hopefully with their good customer service you'll sort out with a good solution.
In Any case, I would say that having these power amounts clamped into a downtube that's far to be parallell to the pulling resulting force of the Chain is not that perfect setup, Mechanically.
There is also another evaluation to do....: at these days bike companies making downhill machines are looking season by season at lighter frames, that means, using Alloys, that the tubings, hydro-formed or whatever they do, are really decreasing the wall thickness mostly far from the critical areas.....the medial part of a down-tube is typically one of the thinner areas for butted or hydro tubings, so not the best point to clamp a drive able to bend Thick Plates like your AFT does.....not mentioning the Carbon ones. As an example, making our own frames, I know well how our pipes are made, And I would never clamp anything to the medial part of one of the Butted Easton PIpes we used for our old XC, frames.....same as for the CF or the Ti pipes. (about 1mm thick)....Having a frame with non round and hydro tubings, means without any doubt an even more specifically engineered variable thickness related to the stress forces the Bike (in its original form) will be subjected to....among which, such a clamping force is not included......
Using a drive made by Matt and looking at his applied concepts and solutions, I realize better also how important is to Have the primary transmission solidly made by a single billet (or in 2 but really firm as a single one) so that the drive itself couldn't move at all, and all you need to look at, is a solid mount to the frame. This means also that the primary alignments are not to be checked and you've not to be worried by that, putting all the efforts to have a properly tuned final stage.
I believe that having FR/DH bike, typically ISCG or other kind of fixed mounts, Using them for the BB mount Is a Must to do.
The pipes where I clamped My Davinci, have been all small round sections like your one. some alternate layers of rubber and engraved alloy sheets (1mm thick) have been of true help....but Again My clamping tube is parallel to the chain pull, so the slipping force is almost 0 and the drive touch the frame to stop any of its small play, plus As you well know the DaVinci Clamps are wide and strong as a common, even quality commercial clamp, couldn't be, spreading the forces into a wider pipe's area and better adapting to shims and rubbers..
Hope to see you finally happy enjoying your e-bike systems...Plus I know that the "true happy rocket system" is yet sitting on your table looking for a proper frame.....I think that a bunch of them will be available soon from multiple sources