Like, they rotate as a play feature? Interesting. In any case, it seems they haven't really been shown in their most flattering position.
You're right of course about the difference between the Rebels and ROTJ A-Wings. Ordinarily the sides of the ship (white in the classic design) have curves, while the centre part (red) is a straight wedge, but Rebels makes that rounded as well. So perhaps curved bricks are more appropriate here than they have been before.
Width though, I'm still not sure about. From the right angle, the movie ship is much slimmer than you'd otherwise give it credit for, but again Rebels changes things and makes it wider and flatter. (Which, just on the side, is curious because the McQuarrie design they nicked the colourscheme and guns from is actually sleeker and longer than the final one.) It's strange, on paper Lego seems to be doing the right things but I can't help thinking the actual model looks off.
Anyway, I do like your MOC. It might be the colours, but it looks a lot more tied together than the set. Fewer studs probably help too.
You may well be right about the engines, I hadn't picked up on that. Definitely too far apart, which ruins the ship's arrowhead shape a little. I wonder if that'll be possible to fix.
The fins look fine on the box art, but at least in the photos I've seen from NY Toy Fair, they're much flatter than that, 45° or more. Hence me blaming the people building the sets, not the designers. But if engines and fins do rotate (Maybe for landing/putting the ship down? Can't spot any landing gear on it) that should indeed not be a problem.