Hooray! I went back to this today, determined to complete the two levels on Advanced that I was stuck on (“Schadenfreud” and “King of the Mountain”), and after $hlmun of goes (especially on Schadenfreud), I finally did it. OK, so they both have completion times of 20+ minutes, but still. So that means all the levels are done, and the game is complete! I also got a measly 25 gamerpoints for my trouble.
After that, I went and finished the two remaining non-under-par Intermediate levels in under-par times. Another 25 gamerpoints, ta very much! That makes 270 overall. Which is crap, I know.