Apparently, for some unbeknownst reason, I was wrong.
After wandering around Todelmer a bit more and, to be relatively spoiler-free, simply retracing my steps and fiddling with whatever it was I'd already fiddled with, I came across something which I assumed was a Bahro symbol or something, but which the game mistook this for the final Keep symbol and promptly linked me so close to endgame it was frightening. I was rather pissed to have essentially skipped an entire age, and so I went back to a saved Laki'ahn game (I unfortunately hadn't saved within Todelmer beforehand) and linked over to Todelmer in an attempt to do it properly and honestly. Of course, this time I managed to run into a bug that jams two rather important levers in the age, and so I was stuck firmly to the ground no matter which saved game I tried to retrace my steps in.
Fortunately enough someone had experienced this bug before and suggested a clean reinstall, but the problem is persisting despite it all and I think I need to either wipe or hide my save files and/or ~/Documents/Shoomy 01 file and I... Gah. DIE. All of you. Especially Esher. I just wanted to play the damn game for the first time in months, but instead I'm sitting at my computer at two in the funking morning watching sound files decompress and drawing angry pictures of myself kicking things.
Rarr. If Adam comes up this week and this remains unsolved, I'm stealing his Myst V. I need my fix.
-C, who apologizes to those in the audience who aren't huge Myst geeks.