Update: SOLVED!

Here's what happened - the issue was linkage related as I suspected. I disassembled the shifter assembly to expose the shift lever itself - it has two points of connection. One (which is visible as soon as you take the boot off) transmits left and right movements of the shifter to the transmission via rotation of the gear selector mechanism inside the transmissions (i.e. "chooses" 1st or 2nd, 3rd or 4th or 5th or 6th depending on which gear pair is selected by the front-to-back position of the fork). This connection was fine. The second is not visible - you have to disassemble the whole shifter and remove the foam insert in the floor to see how it works. See the photos. The front-to-back movements of the shifter are transmitted via a linkage mechanism that sits below the floor (but above the output section of the transmission / driveshaft). The shifter lever has a circle with a hole in it, into which sits a horizontally-mounted pin / shaft from the linkage assembly (piece #20 in the diagram). When the shifter is moved back and forth, the pin moves the linkage and transmits this movement directly to the transmission internally (i.e. moves the selector forks front or back and selects particular gear pairs - 1/2, 3/4 or 5/6). There is supposed to be a clip on the end of the pin to keep it from sliding out of the hole in the bushing at the bottom of the gear selector. This clip evidently disappeared at some point along the line so when the gear selector was moved all the way over (into reverse) it was able to pull free.

I was able to reach into the hole and re-insert the pin so now the transmission can be placed into any gear but to prevent a recurrence a new clip was needed. The good folks at pelican have suggested a clip (actually for automatics, but appears to be the same size) which should be here in a day or two.

I'm very happy that this ended up being a $3.75 repair. I can guarantee that if I'd taken the car to a transmission shop they'd have said "oh it's an internal transmission issue - we have to pull it all apart or replace it" and probably quoted $3k or more. This is why I like doing things myself. Can't wait to get it all back together and out for a spin soon! Maybe an AMG v6 engine upgrade on the horizon too... Thinking about it...