Aw, jeez... Of course. My doofus game is strong.
I actually have completely removed the big cap, spring, and inner and outer plungers. It didn't really try to launch; just a little compression present. In any case, no change: 5th and reverse still not accessible. And I've reinstalled them, careful to ensure they're oriented per the diagram, which seems to match your description:
Looking at that main selector detent on Realoem's diagram, I don't think anything there would explain the issue. The selector rod definitely hits -- and then travels through -- those up-and-back detents across neutral. The grinding I encounter when trying to push into the reverse gate doesn't "feel" like it's at the shaft itself, but rather at the end of the "shift" (as Realoem calls the v-shaped part at the end of the selector shaft). And although there's no grinding when I go for 5th, it is a solid, unyielding wall. (My "temporary success" must have been 4th, rather than 5th, in retrospect.)
I think the reverse/5th shifting rod and/or fork must be stuck. So, the decision I have to make is whether to continue to disassemble or take it to a pro. My big hesitation about disassembling it myself is the possibility of damaging some NLA part. All I've really exposed so far is the input shaft bearing, but that was enough to give me sweats.