No...impossible.
You can not force Horford to do anything. If he opts in, he is here next year at $30 million. And no cap space.
If he decides to stay, he isn't opting out to a 4 year $60 million deal. If he opted in he gets $30 million, so basically he is signing a 3 year $10 million a year extension. That isn't happening. A renegotiated contract for Horford is most likely a 3 year $60 million deal or a 4 year $70-72 million a year deal. Even so, no matter what you do, even your deal, Horford at $15 million still doesn't open max cap space to sign Durant.
In the Durant/Moore for Hayward and Tatum, you really are not opening up much cap space, none if Davis doesn't waive his trade kicker. So as long as Horford is on the team, doesn't matter who's cap holds you renounce, there isn't $38.15 million to sign Durant.
Only if Horford opts out and walks, you renounce everyone, you do your trade and Davis waives his no trade clause do you approach being able to give Durant the max.
Here is the thing? How are you convincing Durant to sign here given it's not NY, LA of SF? Why does he sign when really the only thing linking him to Boston was his great friend Kyrie that just left Boston?
This scenario is logically impossible. Technically, it could happen, but technically a meteor could hit the Earth tomorrow and end humanity, I could hit Powerball and President Trump could be revealed to be having an affair with Nancy Pelosi, but I wouldn't count on it happening