Thanks for all the suggestions.
Loctite red on the throttle plate screws is a must.
My throttle plate valve things are not yet soldered shut.
I should probably take care of both soon.

My return springs are proper, although #1 broke and was replaced with an off-the shelf replacement that was bent into shape. It has good tension and snaps the plate back very quickly. If that were the issue, I think it would be happening more often and consistently

I did have a bad vacuum leak and a funked-up PCV system. One I got the vacuum ports on the carbs closed up and the PCV properly hooked up, my carbs idle smooth and low.

SO.
Once I revved the motor under the hood a few times, the problem vanished and has not (yet) returned. I've not actually yet lubed anything up, but my commute today was fine with a mix of highway on both destination and return. The only difference (aside from me snapping the throttle shafts a couple times), is that it's warmer here today).

Regarding my throttle shaft seals. I had a pair of proper seals split on me while I was troubleshooting, so I built new seals using an O-ring kit and a few small copper washers from Harbor Freight. I used two small O-rings that fit snuggly over the shaft and a slightly larger o-ring that fit between the shaft o-rings and the throttle shaft "tunnel" in the body of the carb. So three rings in all with a smooth copper washer capping off the stacked rings. Aside from a possible role in my sticking throttle, they are working very well.