if Houston decided they were giving up on D12, I don't think it'd take too much to get him. Probably something like this:
D12+Filler for Sully+Rozier+lottery protected 1st Rounder+Lee
We get Dwight and try to make a strong playoff push.
Rockets get a talented frontcourt player who can start or come off the bench (depending what they wanna do with Jones/Capela). They get a feisty defensive PG who, despite being raw, will probably do a better job than Lawson. They get a pick, which can help them in their reload. Lee will make the salaries match and provide cap relief.
I like your trade but I don't think it would take that much. Basically I think if the Rockets get a 1st rounder for Howard they're be happy. I think Sully and Rozier don't need to be involved.
Rockets - Howard and Harrell
Celtics - Zeller, Lee, Jerebko, Dallas 2016 1st, one of our 2016 second rounders.
We do it this way, not giving up the young talent we're already teaching, plus clears up roster space now and future. The Celtics have to many draft picks as it sits with I believe it's 7-8 this year depending. Rockets get 2 draft picks, Zeller who they could resign to be the back up center for them probably for cheap, Jerebko can leave at the end of the year or stay depending on the team. We do this getting Howard who could leave and give us the cap, or can play out next year and still give us room to sign a max player. Harrell is a young power forward who could play excellent in our system, and is signed to a 3 year deal at a million a year. Plus gives us a chance to decide what to do with Sully.