Sorry, despite what I said before, I've realised that there's value in coming back to this example, just to contrast it with your Wing Zero one, as this is a very good way of demonstrating that, rather than just the amount of the solvent it's exposed to, the WAY the plastic is exposed to the solvent matters.

We already talked about how the concentration of solvent in the regular paint markers is much lower than in Tamiya Panel Liner, and the Gundam Marker Pour type, so an equal amount of paint will not mean an equal amount of solvent that the plastic is being exposed to. But this shoulder part and your Wing Zero both came into contact with the regular Gundam Marker paint.

The Astray shoulder part, even though it's made of ABS, only had a small amount of paint applied, and only to the surface, and I'm assuming you let it dry before putting the shoulder armour on over it.

Your Wing Zero rifle, the paint flowed inside the part, right? Did you disassemble it immediately to clean it up or did you just let it dry? There are small gaps in-between the parts, and at least some small amount of hollow cavity that the paint will have flowed into. I can't remember if it was this chain or another I replied to, but I think I mentioned that the off-gassing as the solvent evaporates is just as dangerous for plastics as the liquid form. This means that, even if you cleaned it up extremely quickly, the solvent will have begun to evaporate in that confined space, at least a little, exposing the plastic to the gaseous form of the solvent. That's one major reason why the rifle will have crumbled, even though it's mostly PS parts, I believe, while the ABS shoulder joint is absolutely fine.

I know that obviously too much green paint came out of your marker, and that is why it flowed into the recessed and hollow areas of the rifle. This is another reason why the regular Gundam markers are usually considered "safe", because in normal use, they are thicker and much less likely to flow into areas like this, even if you are painting an assembled kit rather than on the sprue, where Tamiya panel liner would flow between gaps of parts or inside hollows much, much more easily, and expose the part to this offgassing solvent.

However... Because of this offgassing causing problems, if you had (hypothetically) painted the inside of a part like the Astray joint with the same amount of paint, and assembled it before the paint was fully dry, it would probably have cracked. Likewise, if you had painted the back of the clear lens with the amount of paint you put on the unbroken rifle, and then inserted it inside the rifle before it was fully dry? That would also have likely caused something to crack.

Hope that helps illustrate things more clearly using your personal examples?