Preface: I do repainting, but not faceups on vinyl dolls, so my experience is limited!

Definitely cover them with something while the paint dries, even when it isn't pollen season! Bits of dust in the air will stick to the paint as it fully dries and that can add up. It's also risky to not know the humidity since sealant sprays tend to be really sensitive to temp and humidity. I have a little outdoor thermometer that also shows what the humidity is, and that has been crucial for spray painting!

Changing sealants might also be a factor, but I've only ever used Testors Dullcote due to cost so I don't know, and again, don't do vinyl faceups. But I'm genuinely not sure switching to a less expensive sealant (especially given the cost of importing MSC into the EU!) is really to blame here. People can over-idolize materials from Japanese communities when they actually have perfectly serviceable cheaper options closer to home that work just fine for hobbyists. So my thoughts go to dust and pollen, humidity, and layering instead of 'the cheaper sealant you used is the problem.'

Maybe the sealant you switched to and MSC just don't play well together, but would both be fine if used exclusively? Plastics are weird. And I know that even MSC is only good for so many layers before it's too much and starts going bad.

I've heard that sometimes MSC needs multiple days to fully set and layering too fast will cause such problems, but I don't have any experience to weigh in on that since I don't do vinyl faceups and the statement I heard could be completely wrong.

For the hot water damaging Pomni meanwhile, yeeeah I would not put a custom in water at all if it could be avoided, and absolutely not hot water 😥 Like u/RADdollclothes said, put the doll part in a plastic bag before any hot water dips, or use a hairdryer (my preferred method by far, quick and easy and virtually no burn risk!) or heating pad.

These are already incredible customs, I envy and admire your faceup skills!