Again, not going to keep doing this. This isn't true, and you repeating it to twist what I said doesn't make it true. Simmons got his max because he earned it. Not every player who qualifies for the Rose-max gets it. It is a special designation that has to be negotiated by the player's agent. There's no shame in not knowing this minutia of the CBA, but you and I both know you're going to pretend like you didn't read this at the first opportunity and still act like Simmons got his contract independent of his performance.
No. Advanced stats don't care how good the team is in an absolute sense. They were specially designed to factor that out. You're confusing the direction of potential causation. Players on good teams don't have good stats because they're on a good team. Teams with good players on them are good because the players are good, and the stats capture that. If anything, many stats may overrate players on bad teams, because their benches can usually keep up decent production. If Murray has a better plus-minus than Simmons, there's a very good possibility that it's because the Spurs lack the talent to compensate for Murray being on the bench while the Sixers can field such talent. Also, good players often generate a larger percentage of the total stats on a bad team than they do on good teams, since more players are capable of generating those stats. It's possible that Murray's advanced stats are seeing a bump in this manner now that he doesn't have Mills subbing in for him or DeRozan to share stats with.
In this scenario, the height was the only difference, so there isn't a speed or agility advantage for the smaller guy. Height, in and of itself is huge in the NBA. A taller guy can both play in bigger positions because he can match up with taller guys and he can use his size more on offense. The two players might both have the same skills and motions, but the taller players will have a larger shooting window and thus will likely make more shots. The difference between a 6-5 and 6-9 guy is the difference between a star two-guard and an MVP PF.
Who is they? Do you mean that the Spurs would be willing to trade Murray for Simmons? We have no idea if that's something either side would want. Certainly, you'll see a lot of fans of other teams who still think Simmons is worth way more than Murray. We don't know that Morey would want DJM at all, even though I firmly believe Pop wouldn't allow Murray to be dealt regardless. That same idea can apply to any package. We don't know what Morey would actually take. All we can do is speculate based off what he said, and he said he wanted a franchise player in return. Could he be lying? Yes. Could he change his mind? Also yes. But we don't know if those things are true.