Work,
Here's my thought, never having built anything like this and never having opperated your machine or ever even having seen it operate (can I be any clearer around my ignorance??)
OK, with that said.
I'm still of the opinion that your problem is likely the entire subframe flexing or some subset there of.
You did mention that she works fine until under stress, if it was the bend (which I don't really think it is, at least by looking at the picture), I'd expect it to walk off all the time.
When my buddy had flying squirells in his attic we could not figure out where they were coming from. We took the mini-bosses video monitor, put it on his roof, hooked it up to his VCR and recorded the roof all night long, sure enough, upon inspection the next morning his wife could see them coming out of the ridge vent, hole plugged, problem solved.
I'd suggest the same with your machine. Video tape it, really good, a couple of times showing the chain walking off, I think that may give you the answer.
Would hate to see you fix and re-fix the wrong problem.
With all that said, I hope your solution works!!!!
I think everyone should own a track machine at least once, I've had two and until you've operated one, that itch is never really scratched.
Good luck my friend and thanks for sharing your terrific project.
Sincerely,
Joel