Well, I found the problem !
For the benefit of anyone else who stumbles onto the same symptoms, there's one unlikely root cause you probably hadn't thought of: a broken clutch lever pivot fulcrum. No wonder I couldn't wrap my head around any reasonable cause for the behavior I was seeing !
The plastic pivot point slides into a cylindrical hole in the transmission to locate the other side of the clutch lever arm, opposite the slave cylinder. This plastic piece (likely glass reinforced ABS) appears to have been defective from the factory; the tip looks like it wasn't given enough time under pressure to allow all the air to be expelled from the mold, or the plastic wasn't hot enough. Either way, the the weakened hollow tip had broken off during service, resulting in a full 10mm drop in height from the new part (see pictures).
The sad part is I had never thought the synchros were being tortured the entire time we had been driving it. A working hydraulically-actuated clutch slips long before clutch disengagement is not possible, so it never really occurred to me my gf might be slowly killing the tranny.
Needless to say I'm less than hopeful that the synchros are still top notch, which sucks because this car has only 70k. I've decided to gamble and install it anyway, in hopes the synchros are still good, and if not will pull it all apart again, along with my other 318ti with 210k that I'm planing to sell after all this. I've got my fingers crossed. Wish me luck!
By the way, I decided to go with a Fidanza 12lb flywheel so will be putting a good dual-mass along with a new Sachs clutch kit and flywheel bolts up on the classifieds. It's pretty much everything you'll need from the crank to the transmission for a fresh flywheel/clutch job. The Fidanza uses a 240mm sprung M3 disk and pressure plate so I couldn't use the stock 215mm unsprung setup I bought from Turner
This is by far the most difficult clutch job I've ever done. I miss my 911 in that regard
I'll update on the outcome once it's together.