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Nothing short of "hanging gauges" will accurately diagnose your issues. Perhaps your friend knows someone with a set of gauges and a frame book. The dimensions in the FSM can be interpreted and used by a "talented person" but you still need quality measuring devices so having someone "hang gauges" is the real answer you're looking for.
Might not be that expensive of an operation if your friend knows someone and it's certainly a "learning experience". If you're looking "to buy" there's a good bit to be learned from this experience.
A buy of most anything here gets gauges before anything is done.
Your issue seems to be of substantial "differences" so I'd think you should be able to use the FSM and conventional measuring devices to do a first check.
I'm still curious as to what your actual wheel base measurement is on each side. Do you feel like you turn one direction harder than the other? Is that tire on the "shorter" side wearing any differently than the opposite side? If the wheel base was an inch off, it seems to me that you would be able to see evidence of that in your tire wear.
Haven't driven it but a few miles or so.. Honestly I'm not terribly attached to the car so I believe in going to either part it out or sell it as a roller and maybe someone else wants to go through that hassle. I'll put my engine and transmission I built in something else.. Maybe another c4 if I can find one..