Hey, Doork?
You truly have no idea how the movie industry works. None. You don’t even know what “final cut” means if you say “She submits her final cut to the studio, which they can change a little or a lot if they like”. No, they do not. Not if she has “Final Cut”. If she has final cut, and it meets typical release-quality standards (like it gets the rating called for in the contract (see below), etc), then she has final cut. There’s no “they can change a little or a lot if they like”.
Also, all this shit works via contracts. You just don’t call up people who have fulfilled their contracts, and say “we need to shoot a final scene”… unless that’s in their contracts. Otherwise, you’re screwed. And you sure as hell don’t do this after the movie has been released. Why? Even more importantly, How? All those people are gone, working on different projects! They don’t care about your “New Zealand TV needs 3 minutes of Big to fill out running time, and stat!” desire. LOL.
The “second unit” is not a group of guys sitting around waiting for a call - “we need a car chase for Spielberg!” “We need a new ending for Altman!” - at the studio’s beck and call. No, the second unit, most often, is used for shots that do not need the director’s attention - establishing shots, insets, more.
Just as with the SGA, the DGA would frown heavily upon using a second unit to replace the first unit directors without a corresponding bump in salary, billing, more. Since this didn’t happen either, it’s safe to say the “second unit being called to film principal photography” theory you espoused is, again, pure fantasy.
Lastly, Penny Marshall in 1987 wasn’t merely a “2nd-time director”. She and her brother Gerry Marshall made themselves to be power brokers by the early 80s - him first, with his creation of Happy Days, Mork and Mindy, Laverne and Shirley and other shows for ABC, then directing such films as Pretty Woman, Beaches, and more, her as an actress, then director. A typical power couple, only this one wasn’t married.