Okay, so one day with the tiny air hole plugged, and leaks have actually been
much improved (not gone). The only real leak I got was after a night leaving the mod/tank upright, and even then the liquid didn't actually drip out of the vent, but was clearly pooling there.
I used a "metal pick" thingy that's actually sold as a blackhead remover (
https://smile.amazon.com/gp/product/B006Z9UPQO). Despite its intended use, it's the most useful little general-purpose tool I own for tiny intricate things.
I used that to poke a tiny rice-grain-sized bit of cotton into the tiny air hole in the tank. It took a few tries to get it to where it was pugging the hole but wasn't sticking out the top, cause otherwise that blocks the tank's top from sliding back into closed position.
I've been testing this with the fishbowl glass, which is again usually more prone to leaks in my experience. I have
not been doing dry pulls when I refill, just cause I want to see just how much of a difference this hole-plugging alone will make.
So far it's seemed to be pretty good, but I'll need to use it this way for a more extended time to really say. I will update y'alls in a couple days again.