The script for The Voyage Home was not a mash-up, but rather a result of Meyer and Bennett collaborating. There is definitely a tonal difference between the acts, but it's not like they were acting independently of one another.
Right. A lot of collaborations entail the writers each taking a different subplot or portion of the story as an agreed-upon division of labor -- then rewriting each other's scenes to give it consistency.
Lets remember how bad the first two seasons of TNG were when Gene was the showrunner, he was still doing the show like it was stuck in the 60s!
Except with a lot less censorship.
And actually Roddenberry was only the showrunner in season 1. It was Maurice Hurley in season 2.