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.
Correct. The "mash-up" script was ST II
- where Harve Bennett pulled in one pitch about a weather machine, another featuring a young male Vulcan called Dr Savik, another with Kirk's ex, Dr Janet Wallace and their son, David, and a sequel to Khan's story - and smooshed them all together.