We're not getting a lot of attention from others on this thread, are we? haha. It's possible that my sense of humor has alienated everyone else. :-) Regardless: thanks for your time, Monty.

So, I DID, in fact, skip a gear. I dropped out of 5th and was aiming for 3rd, but couldn't get to it. So it seems this is where I failed, and caused this failure. I thought that because I was at the bottom of 5th, and shedding speed so fast in the braking zone that I could get to the top of 3rd, rather than bump into 4th in between. This seemed to make sense to me, since middle of 3rd is where I wanted to be as I drove out of T1.

So teach me: why does it matter?
Thinking out loud here: If the motor is turning at 7000RPM and the trans' input shaft is turning at 7000RPM, then we shouldn't care if it's 3rd or 4th, right? EXCEPT, of course, if our speed is wrong for that RPM in that gear.
So what we're REALLY saying, is that I was going too fast to try to get to 3rd. Which I think means I was going over 105-ish. Which suggests I was either downshifting too soon in my braking zone, or I was traveling much faster this lap than I thought I was. (This was the first time I'd reached 5th gear on the straight, but I sort of short-shifted, thinking I could get into the meat of the power. maybe it worked?) If this is the case, then no amount of "Rev-matching" was going to get me there: I was simply traveling too fast for 3rd gear, so when the synchros engaged, with the clutch disengaged (no clamping pressure) the centripetal forces simply overwhelmed the stock-type clutch disc.

Did I make that leap correctly? in which case, I need to brake more before I try to stab at 3rd gear, so I don;t do this again. OR: do I need to bump from 5th, to 4th, to 3rd every time, regardless of speed?

IMHO you are spot on. In 44 years of driving mostly 911s the one thing that is constant is the shifter has always been a vague mechanism. I highly recommend going into each gear while downshifting.

PS I don't know if Sachs or any one else does testing on their clutches to destruction. However you must consider that manufacturers are doing everything they can to reduce clutch disk weight to reduce mass and inertia in order to speed up shift speed and reduce wear on the synchros.

Sorry removed I miss read you first post.

You should call CA Motorsports, https://californiamotorsports.net/pages/contact-us.

They can probably tell you what happened & why. Get their Billet shift arrester it makes shifting much smoother & they can help with your next trans.