So I can kind of rest assured knowing that the Part III that exists is the best possible Part III that could have ever been made, short of Duvall not appearing in it and Sofia Coppola playing Mary.
Pretty much agree completely, although I think Tom Hagen's presence is a bit more prominent than you suggested, and would definitely have helped the finished film.
The Vincent/Mary subplot in this script is even messier & less credible than what we ended up with - they have a sudden meeting at Vincent's HQ & the romance starts. I had to go back & check to see if they'd met at the party or anywhere else, but no.
It's a very rough draft that needed a lot of work, you're quite right about it being 'lazy'.
