He won't help us pass our way out of defence, which we're largely in agreement that we're not very good at.
He certainly will. Fellaini offers a great target for defenders to give the ball to and excels in taking control under pressure and bringing the players around him into play.
He won't give us added control in CM when pressurized high up the pitch, which we struggle with.
Again, Felliani excels on the ball under pressure.
He won't give us any dynamism in a midfield two, and won't help us connect our central midfielders to the rest of the side.
I'm not sure you really understand the role of players such as Fellaini. His reliability of control and strength under pressure means he can basically act as a target man in the middle of the pitch. This allows all the players around him to get forwards quicker and with more confidence as Fellaini holds the ball and looks for the lay-off.
He won't help us play through the middle of teams due to his incisive or creative passing.
No, he'll help us play through the middle of teams through his strength and holding ability.
He isn't particularly aware defensively, and we've got Carrick anyway for that, so he won't really make us that much more solid. Sure, he'll clatter into people and throw some elbows, but in terms of us looking more cohesive as a defensive unit - no.
In the defensive phase he meanders around our own half making himself a right nuisance around the ball. He's a big unit and he uses that physicality to make sure that the opposition is always aware of his presence, often blocking the way to the simple pass and forcing higher levels of creativity from the opposing forwards. He lacks pace and can be played around, but in so doing he forces the opposition into channels whereby our other defenders can deal with the threat. Defensively he's a huge boon.
What he does is he gives us an option when Carrick doesn't play, which is obviously dearly needed, and he does give us an option as a goalscorer from midfield.
You clearly don't know this player very well.