I honestly think the grade still needs to be incomplete until this offseason is finished. Horford's contract should be easier to trade than Kemba's this summer, due to it being $23 million less. And if he's not traded, stretching $14.5 million is a lot less limiting long-term than stretching $37.5 million. Any of the choices you presented that involved keeping Kemba would have put the Celtics above the hard cap this summer, limiting the moves the team could make while having an even worse team than they do now.
No clue where it will end up, and honestly there's no good answers when you owe two years, $70+ million to a guy who can't even perform at a reserve level. Given what has happened in the last season, my preference would have been trading Kemba and a lotto-protected 1st for Horford at the deadline last year, and skipped getting Fournier. Then we could have had a max-level trade exception ($34.4 million) this year, and otherwise ended up in a similar place. Seems like getting Fournier was Ainge's last bet to go out a winner. Given how quickly Stevens made the trade upon taking over, he might have pivoted faster.
This is largely true, although with the path we chose, I don't foresee circumstances that would justify giving up the #16 pick. I think it's unlikely we acquire anybody in sign-and-trade, but I guess time will tell.
One thing on the Horford contract that you understand, but I think many don't: only the amount of salary that we guarantee is used for salary matching purposes. A lot of folks seem to be under the impression that we can trade Horford's $26.5 million for a player making $33 million, with that player's team immediately waiving Horford and only having to pay him $14.5 million guaranteed (or $4.83 million per year for three seasons.) Under that scenario, a team could immediately clear $28.17 million in salary.
But, that's not how it works. Only the guaranteed salary counts. So, as is, we can bring back somebody making $18.225 million ($14.5 million x 1.25 + $100k). A team only saves $3.725 million unless it stretches Horford. And, that's if they cut Horford, meaning they're trading a player who would have played for a player who won't be on their roster. Now, presumably the team and Horford could renegotiate to guarantee more of his salary, but that also results in less savings for the acquiring team.