More realistic scenario:
1. Celtics let KO, Zeller, Amir, Jerebko, Young, and Mickey walk clearing cap space
2. Sign Hayward to a max
3. Trade Bradley, Crowder, 2018 LA pick, for George
4. Use room exception and get min contracts to fill in the frontcourt with PF and C types.
Without the extension (and I prefer the deal without the extension since I have no interest in dealing JB and Smart/Tatum to make the necessary room), I am hoping we can hang on to the LAL/Sac pick and maybe deal the Memphis and either the Clips pick or our 2018 pick. That is still better than the Lakers are giving up, especially with AB and Crowder in the deal. If Indy doesn't want AB, a third team can get involved and send another first to Indy.
But, yes, your general plan is what I think the majority of Cs fans are hoping for at this point, although I will question Horford's future after this season with the contracts of IT, George and Smart expiring. If we can deal him for a cheaper, yet totally viable big, that is the only way the luxury tax doesn't reach record territory.
Your plan also involves Hayward taking slightly less than the max since the new projected cap came in at $2M less than first thought. I suppose Rozier can be sent off in this scenario.