1. It's mentioned in passing on Leige Maximo, but the point is it's mentioned. The movies haven't explored enough on megs and the fallen to make any real claims one way or the other though on what's going on with them. All we really know is that megatron leads the decepticons and bows to the fallen for unknown reasons. Shockwave in this movie could bring that to light though. 2. Starscream filling the leader void, and not trying to restore megatron would be a scheme, but it also boils down to who you think shot megatron in movie 1 with the missles. Schemer maybe not so much, but he still has his whole "I want leadership." Cowardice, and mild plotting. Though to be honest even in G1 his plotting was mostly "I'll just shoot megatron in the back!" and then cowardly beg for forgiveness when it doesn't work. The little details including megatron snapping at him are what insinuate the details of their relationship though. It's like saying Megatron kills prime for no reason. There is a reason, it's just not spoon fed to the audience, unlike Prime, it's just not as obvious of a reason. It is rather interesting when you see his 'oh shit megs is back' reactions on the ship though. 3. Soundwave really couldn't come down to earth, or they'd lose his information gathering, or misinformation he fed to the good guys. If he came down to earth at megatron's side. The entire final battle scene wouldn't have made sense as to why there wasn't a massive global strike on that spot.