Agreed about your opening thoughts. I almost feel we’re misdiagnosing the problem with the opening. It moved so fast it felt like whiplash, so we feel it needs to be improved by extending it. Yes, having Mustafar deleted scenes would have helped, but since we don’t have any I think keeping that Mustafar opening is just a detriment to the film. I think it feels like whiplash because we get an information overload within a short period of time. Palpatine is revealed to be back in the opening crawl, with no build up whatsoever. Then we pan down to a planet, then immediately jump to the surface, no ship transition, which is normal for literally every other Star Wars movie. We are there for 30 seconds, then we transition back to space. It’s just a lot.

I agree that starting with Kylo already in possession of the wayfinder would probably work better. Either with him flying into frame from left to right, heading toward the nebula as he starts his journey through it, or pan down to that nebula with Kylo jumping out of hyperspace. The crawl could set up a little mystery, maybe that the Wayfinder belonged to Snoke (which makes much more sense that Vader having it).

I sympathize with wanting to delay Palpatine’s reveal a little, but it seems the filmmakers decided to push it forward because they felt that the stakes needed to be introduced early on in the film. With no deleted material to play around with, I think it would be better to stick with that approach.