I think the Rockets prefer to have the ball in Harden's hands at all times that's why they moved him to PG.
Ball and Simmons is redundant and I don't trust his jumper. Fultz and Smith are ball dominant. How do we spread the ball around between Simmons, a ball dominant guard, and Embiid who needs low and high post touches and P&R/P&P's ran.
They have the ball in his hands because the alternative is Beverly, and he's PG on offense, defensively Beverly guards the PG. When D'Antoni had Nash and JJ, he allowed Johnson to play on the ball at times and create. There's nothing about Mike's system that requires the ball to be in 1 player's hands only.
As for how you spread it around, same way teams like the Cavs, Warriors, Bucks etc spread the ball around. It's not that difficult especially when I really don't see Simmons being a dominant scorer, the Lakers used to share the ball between Kobe, Pau, Odom and Bynum. With Bret Brown being from the Popovich line and having coached with the Spurs during Manu/Parker/Duncan's primes I'm sure they'd be able to figure out how to get two ball dominant players and a big to work together.
I like Fultz or Smith more for them than Ball, but Ball has some positives if he's paired with Monk in that he'd be running the offense from the 2.