Beyond the CG footage sticking out from the following live-action sequence, there’s a few other reasons why I don’t think the battlefront footage fits in.
Compare this to the opening of TFA. We see the transports leave the destroyer from space, and we later see them land at the village. We follow these ships to the action. These Mustafar shuttles have no introduction and we cut to them jarringly, never to see them again. As others have mentioned, we also see a Star Destroyer in-atmosphere, even though the only Star Destroyer we previously saw was in orbit. These shots imply that we are also witnessing the beginning of this battle, but when we cut to Kylo Ren it appears as if we are in the middle of the battle already. Also, the normal speed of the battlefront shots doesn’t gel really well with the slow-motion Kylo’s shots.
These shots thrown in where they are arguably makes this opening sequence more erratic than it was, imo. It’s not that I don’t like it in theory, but I don’t really see or feel how these shots flow into one another. It’s why I didn’t like how the original jumped from space, and immediately into the middle of a battle.
That is why I like the shot of the TIEs flying over the battlefield. We get visual continuity between the space shot and the landscape shot with the TIEs flying in Vic formation. Perhaps you could create even stronger visual continuity between the landscape shot and the first ground shot by adding small yellow and red blaster bolts zipping across the battlefield, since we also see the in the following shot with Kylo. Adding element of battle, like blaster fire and explosions, might help alleviate that comforting feeling you also described that shot having. We see that we are already in the midst of battle, which lines up with what we see in the following Kylo sequence.
To me, that flyby shot is the only shot we’ve experimented with that genuinely feels like it could’ve been plopped right into the theatrical movie. I just personally don’t think the battlefront shots hold up from a visual perspective or an editing perspective.
I hate saying that because I know other editors put a lot of time into making those shots fit better into the scene and I think they did a great job at that (my problem with the footage definitely isn’t their contribution to it!). This whole Mustafar conundrum has been a long process of experimentation and trial & error. But it just comes down to honestly figuring out what works and what doesn’t. And out of everything this what I think makes the most sense.
TL;DR: RL is a stickler for visual continuity.