Maybe it changed in the release version, but for my VEN game it was pretty rare for those I forced into my league to leave it later.
ENG is probably not joining due to your relative sizes, bigger nations are less keen to join. You could always get a different misison when canceling though, is that not happening?
ENG is probably not joining due to your relative sizes, bigger nations are less keen to join. You could always get a different misison when canceling though, is that not happening?