The GeForce4MX is a Geforce2 derivative, which means that it has only 2 texture stages (the renderer should state something like this when initializing). That means that 2 textures can be processed in one pass, which is what jPCT does. When adding another texture, jPCT falls back to multiple passes, i.e. rendering each triangle multiple times, which is why the size increases and rendering slows down significantly. This is not a bug, it's a feature.
The first problem is related to this (or so i think), because when adding another texture, the preconditions mentioned in the Config-setting aren't met any longer, i.e. enviroment map isn't on stage 2 anymore, which basically means that in this case, you would have to revert the envmap to stage 1 again. But the application has to do this, i can't do it automatically.