As to Ainge, this team was built primarily through the draft, Ainge's drafts. There is no debating that. Ainge gets criticized here all the time for any and all bad draft picks or misses (and he has some, no doubt) but this team was built through the draft. Picking Brown was criticized plenty and most everyone would have been happy to draft Fultz.
The team was then rounded out first with the Horford-Walker trade which likely did have a lot if input from Ainge. It is Stevens' trade though, it happened on his watch and so he gets the credit. This trade was mostly panned. The most common remark was I can't believe we had to give up a draft pick to dump Kemba. I admit, I did not expect Horford to be this good but that trade now looks brilliant.
And then the final trade deadline things that got White and Theis. Not a huge impact but moves that have proved helpful.
Then here is the coach. Yes, Ime is a good coach but I am not sure the team is any better right now than it would be if Stevens was the coach. Stevens got teams to the ECF so no reason to think he couldn't have gotten this team to up 3-0 on the Nets.
So what does this all add up to? What did each of these three contribute:
Draft 80%
Horford Trade 15%
Trade Deadline 5%
Coach ?
These are just rough numbers off the top of my head. So I give Ainge 80% of the credit for this team.