This is indeed a difficult call. I hope to have some quiet time this weekend to consider the issues in depth. I'm still concerned about the irreversibility of merging content from existing forums, even though I support dropping those forums.
I very much felt the same way.
However, we could do this in baby steps: first create a general rental car forum, and then after some time examine the individual rental car agency fora and determine whether some of them should be closed or merged. The problem is that merging the four targeted fora is basically irreversible, although labeling threads with the specific rental car agency name would help to alleviate that problem and also help with the concern about having the four rental car agencies together in a single forum.