Glad to see you're enjoying the new molecule version of fix GCMC. I hope that you and other users find it useful.
You're right about the volume needing to be adjusted when using the region option. I'll change that.
On the GCMC acceptance criteria, you also make a valid point that in the literature, the formula is more complicated when dealing with molecules instead of atoms. However, it isn't immediately obvious to me how much difference this change would make in our context, what the exact formula should be for molecules in our case, or even how to compute all of the necessary terms. I'd welcome your help, or the help of others on the list, in tracking down answers to these questions.