With respect to Smart's shooting, you must have missed the below.I hope not. A guy who can't shoot from anywhere on the floor is not a good option to lead the second unit. He's a much better option to start, where we can hide his shooting in the corner (where he's at least average), and he can be free to wreak havoc defensively.
Celtics will run the ball through Horford a lot when IT sits, I would guess, with Rozier running some pick and roll. Rozier isn't a much better shooter than Marcus, of course, but he's at least shown the ability to get to the rim.
Smart has been leading the second unit for a while now. As of right now, he's probably the best passer on the team other than GH, IT4, and Horford, all of whom are going to be starting.