Ugh!
...merging layers is messy for sure. Thing is, Verve's layering system is totally unorthodox to begin with. Good thing you have figured out a way to deal with it. Everything I'm learning right now by developing Vervette is teaching me a great deal no how I can approach layers differently, even with my wicked "material" channel. I've figured out a bunch of things already that will improve Verve a great deal.
However, it's a whole lot of work ahead and i will have to get to it at some point, but can't right now.
...but once I do, oh yes, things will change massively!
I'm not sure what's going on with the drying, but normally it would stop changing, if you have "drying" turned on! But Verve was full or little weird things. If fluidity is turned off, it should never change by itself at all, of course.
The only other thing to consider is: If you have any amount of fluidity on, the moment you make a stroke, it will transfer energy into the paint, which will propagate into all connected paint, but shouldn't transfer across dry/empty areas. The stronger/faster your stroke, the more it will spread, of course, even at low fluidity. The energy that goes in will move through, even if only by tiny amounts.
In Vervette's Sandbox I started out with having fluids stop automatically when you end your stroke. I loved the behavior, because it is 100% harmless with no chance to do more than you would expect. One has to learn to work with it a little bit, but it feels great, I find.
Maybe I will add such a functionality into Verve, too. Would make everything more solid.