Houston has leverage because they don't have to trade Harden and since, to my knowledge, he doesn't have a no trade clause, they can trade him to whomever they choose whether that team is on his list of acceptable teams or not.
The Rockets also have leverage because he still has 2 years left on his deal. So they don't really have to trade him at all.
Right now, the Rockets are playing a bit of a long game and waiting to see if a borderline playoff team or borderline contender gets desperate enough to give them the return they're looking for, or at least get close.
Harden's playing it pretty stupidly, imo, because what he's doing isn't going to encourage teams to increase their offers, which means he's stuck in Houston. The Rockets have made it pretty clear that they are looking for a deal similar to what the Pelicans got for AD.
As for Simmons, other than the expected improvement that comes with experience, he is still essentially the same player he was as a rookie. His unwillingness to shoot the 3 is a big detriment because teams don't have to respect his shot, which means they can play off of him on defense and make the rest of his game more difficult.