TOPIC:

At idle, the slides should not move at all. Are the springs in there?
But, it's possible you are idling on the main circuit. That would cause the throttle plate to be open further than normal at idle, and that could lead to the slides bouncing up a bit. Idling on the main (or intermediate) circuit would cause poor atomization so that might explain the splash back of fuel.

See if there are any wear marks where the throttle plate would be for normal idle and see if you are indeed at those marks with the throttle plate.

Idling on the main is usually cause by gummed up pilot jets, or air passages, or mixture screws out of whack, or vacuum leaks.

Awesome. Thanks for the lead.
Slides were definitely bouncing at idle, but they spring firmly back down with engine off. I'll check springs, and re-clean pilot jets, air passages, and read up on what a throttle plate is (I assume it's the butterfly). I'll be looking for wear grooves.
The only mixture screws I have are the idle screws, so they'll get extra attention.
Further updates as events warrant.
edit: Went with an orange-black theme for the new paint in reverence to loudhvx. New profile pic up next.