New proposal:

Celtics end up with:
Smart/Harden/Brown/Tatum/Thompson
Teague/McLemore/Young/R.Williams
Pritchard/Langford/Green/Ojeleye and 2 open roster spots (O'Quinn + buy-out?) and 5.5M to spend below the tax cap.
I’ve been thinking of something pretty similar:
To BOS: Harden, PJ Tucker
To CHI: Kemba Walker, Langford, Robert Williams, future pick
To HOU: Zach Lavine, Lauri Markkanen, Aaron Nesmith, Carsen Edwards, two 1sts, & two 2nds, TPE for Tucker
ALSO: The Celtics can do all of this while only using $7.9M of their TPE on Tucker. They could incorporate the remaining $20.6M of the TPE in order to strengthen the return for HOU. For example:
To BOS: protected 2nd
To ORL: Eric Gordon
To HOU: Evan Fournier (on an expiring)
In this scenario, the Celtics trade the TPE for Evan Fournier. The Magic then take that TPE and use it on Eric Gordon. Lastly, the Rockets send Boston a protected 2nd for Fournier. I believe the trade would take at least 19 days to be completed due to trade rules?
Final return for Harden: Zach Lavine, Evan Fournier, Aaron Nesmith, Lauri Markkanen, Carsen Edwards, two TPEs (one for Eric Gordon, one for PJ Tucker) & PICKS
Not sure if CHI is getting enough for Markkanen, but they would need to be compensated accordingly for this to be possible.