Hi Steve!
Following this with great interest, and appreciate the steps you've taken with video to illustrate the problem. If a photo is worth a thousand words, then video must be worth 5x that because we can see what you're seeing. Well done.
Thinking logically -- so long as the eccentric is tight in its mounts so the lot cannot wobble -- the BB spindle should spin concentrically unless it is bent. If the worst happened and the BB threads were tapped at a cant in the eccentric, then the BB spindle should still spin, but at a consistent angle to the frame, rather than wobble.
If you have a means to check left-arm runout, that would be a second check on BB alignment and runout. Lacking a dial gauge, a quick and easy way it to place a Sharpie permanent marker next to the BB shell or frame tube and used it to measure where/if there is a high and low spot as the crank spins. This can also be done on the right side, marking both spider and 'ring independently. The marks remove readily by wiping with a rag soaked in isopropyl alcohol.
Over the years, I've seen several things ause a wobble such as you have:
• A bent crankarm chainring mounting spider (shipping damage)
• A bent chainring (crash damage)
• Crankarm spider mounting seats uneven due to a thin spacer left between 'ring and spider (assembly error)
• Broached square-taper mount off-axis (manufacturing/QC error)
Given you've checked basic alignment, I think the last might be the problem. Given the overly-tight pedal threads, it sounds to me as if this Middleburn crank might have been faulty.
As for the Thorn cranks' ring-wobble, so long as the BB bearings don't have excess play, the lot should spin evenly. For reference, all my cranks spin "round" well within 1mm total runout.
In no way implying you've missed something, but in an effort to help: Have you checked to make sure the chainring is evenly seated on each spider land, and the peg bolts and sleeve nuts are torqued evenly?
Looking forward to hearing further developments, and hoping very much for a quick resolution so you can fully enjoy your new bike. It is terribly frustrating when things go awry, especially in a baffling fashion.
All the best,
Dan.