I'd more or less stopped using RandomExtra.
I'd been using it because I wanted Fliqlo and Polar Clock (classics, right?) to work in tandem, one on the external monitor and one on the MBP.
Somewhere in there - in Fliqlo, Polar Clock, or RandomExtra - there was a bug that prevented a screensaver process from quitting, which, in turn, prevented the Expose/Spaces shortcuts from working.
I've set this all up again, the way I had once before (only with the most recent versions of Polar Clock and RandomExtra) in the hopes that the issue has since disappeared.
It's surprising that in all of it's options, RandomExtra doesn't seem to have an option for specifically setting a particular screen saver to each monitor. I think the closest you can come is to activate the two screen savers you want, and set the "Pick Mode" under "Extra" to "One By One." This works fine for my purposes, though it feels a bit like a bootleg workaround.
Anyway, here's hoping for the best this time.