This is just me, but if all of my options with the factory ended up being me sending the M3W back on my dime, I would go to my local race car chassis builder and have them take a look at the situation.
The original frame is tube with welds. I'm sure that a competent chassis roll bar fabricator could "fix" the cracked area and reinforce at the same time. Probably for around $500.
I'm sure the engine would need to be removed and the upper right A arm etc, but this is not rocket science and a SAFE fix can be made. This is my opinion only, but I would be looking at Plan B right now.
A fix like this could be done in less than a couple weeks and probably well within driving/trailering range.
I've read this whole thing and a "fix" has always been in the back of my mind.
Also if it were me, I would look at having the same person design a front cross tie brace between the two front upper frame tubes. This can only help to box in the frame for even more support.
I hope you end up with an acceptable fix one way or the other.
