Now that I’ve seen the shot with only the Stardestroyer I think that this is the best solution. It explains why nobody is panicing since there is no attack incoming yet but it also gets rid of Kylos shuttle approaching from a different angle. So the attack isn’t really meant to be a complete surprise attack but the FO wants to hide their true intentions until it’s too late.
Having the Stardestroyer shoot a ship could work but I think it’s not necessary. It would provide a good reason for Finn to look up but I never had any problems with that. You could also add a little crowd noise that makes him wonder what’s going on and so he turns around and sees the destroyer in the sky. And since we have “They found us.” instead of “They’ve done it.” anyway we don’t need a reason for the latter.
About Leia’s hair: It looks like it should be possible to digitally remove the braid which would make it no more noticeable than Kylo’s backhead in the Snoke scenes. But even if it’s not, I think that this continuity error is a small price to pay to have this scene in the movie.