Root Cause Analysis Post:
Reserving this post for the end when I find what the root cause was.
Original Working theory (wrong):
Bent a shift fork, leading to circlip migration, leading to trying to be in two gears at once, leading output shaft shifting forward slightly, moving the clutch basket, leading to the scoring on it.
Current Theory, Supported by damage analysis:
3rd Gear circlip was not in it's groove.
What caused 3rd gear's circlip to jump out of the groove, now there's the question. The direction it moves when it's forced out of the groove is in the direction of a 5th to 4th downshift (3/4 share a input gear), and that is the only normal movement that can cause side loads onto 3rd gear. Side load could also be generated in a failed 4-5 upshift if the dogs bounced instead of engaging when you shifted. I've never had a failed 4-5 up shift, so I can only conclude that being heavy footed in 5 to 4 downshifts is the cause, as there is no other normal action in the transmission that could possibly cause the circlip to get pushed.
Unless one of the much smarter guys here knows something I don't about how this whole apparatus works, I consider the mystery solved.
CAUSE:
Lead foot 5-4 downshifts = dislocated 3rd gear circlip = wobbling third and fourth ouput gear = time bomb waiting to explode when and if 3rd wobbles.