So, I've been recapping while I've been waiting for the various "poor-man's alignment" tools to arrive, and when I finally got around to the FM IF and MPX boards things improved slightly (there are now about 4 stations I can get intermittent "solid" FM STEREO indicator lights on), but I still have the intermittent FM STEREO flickering during AUX, PHONO, AM, etc. which it clearly should not be. I checked the SELECTOR switch circuitry and it looks clean, so I started looking in the schematic for any potential sources of noise... that's what the flickering resembles most... it's not steady, repetitive, or responsive to me probing the antenna inputs, etc... It stabilizes slightly when I probe a couple of test points on the MPX board with a voltmeter, but is still never solid.
Replaced the caps on the ripple filter board, things looked even better. Started tracing signals from the ripple filter board to see where they led... a small board, F1120, which is not in the service manual but is in the schematic. A couple of electrolytics, replace... look, a transistor! Can't read what it says on the side, but check the schematic... uh oh...
Yep. My old friend 2SC458 directly in the signal path from the IF out to the MPX in. If ever there were a (somewhat) predictable source of noise that could be it.
I'll be replacing that now, then aligning with dr*audio's suggestions. The oscilloscope came in and I'm playing with it now. It has a 1kHz generator as a calibration signal, but it's square wave, not sinusoidal, so alas I'll have to use something else to modulate into the FM.
So I'll be playing with the oscope until I get some KSC1845Fs on hand. Probably best to keep some around anyway if I'm going to keep restoring these old delights.