I think you are 100% on about the idea of Ainge using the #1 pick if he got it as a way to attempt to entice Durant. It lets him go to Durant and ask him who he'd rather play with: Ben Simmons or a guy like Cousins or whoever else that Simmons could be dealt for (which is a lot of guys considering the value that a Ben Simmons holds). And it just so happens that in a lot of cases, NBA players prefer proven NBA studs over hyped up 19 year old college kids they aren't totally sure are the real deal and/or how long they'll take to reach their full potential.
See CLE trading Wiggins for Love to get LeBron to come back (and I know that trade happened after LeBron came back, but that was only because Wiggins couldn't be dealt for 30 days after the draft- that deal was in place all along). If CLE doesn't win the lottery, does LeBron really go back to CLE to play with Kyrie and role players? I highly doubt it. Winning that lottery and knowing it would deliver Kevin Love ended up bringing LeBron James back to the Cavs.
If Celtics win the lottery, don't be surprised at all if Ainge tries to pull off something similar with Durant. Whether that's Cousins or signing a Kevin Durant and Al Horford package deal and moving the pick for a Rudy Gobert, who those 2 may prefer to play with more than a 19 year old kid (and most importantly, makes $2M so allows us to save max cap space for 2 guys like Durant and Horford). I expect Ainge to go to Durant and present multiple roster options to him, particularly if we somehow get lucky and win the lottery, which increases our options tenfold.
It's not neccessarily about getting the best value for the pick. In a vacuum Simmons has a lot more value than guys like Cousins (purely due to how insane he is) and especially Gobert. But if it leads to Kevin Durant, you can bet Simmons would be shipped out in 2 seconds just like Andrew Wiggins was. Someone as good as LeBron or Durant always calls the shots in situations like this. They are the FA's, they have the leverage and the rosters will be molded towards what they want the most more so than what the GM would typically do.
I'm also less convinced than others that Durant is a lock to stay in OKC. Yes, they are real good. But they've been real good for how long and still haven't come particularly close to winning a title? Where is the evidence they are about to get over the hump either? Clearly the 3rd best team in West, and GS is super young and going nowhere anytime soon even if SA fades some soon. It's not like OKC will have cap space or quality picks to improve in the future, either.
If they get bounced in 2nd round again (which seems likely at this point), it wouldn't surprise me at all if Durant was fed up of the same thing every year: great regular seasons and super talented teams that just can't beat the absolute best teams in the league. And it wouldn't surprise me if their owners history of shipping off talent like Harden (for a mind bogglingly awful return at that) over luxury tax concerns rubbed Durant the wrong way.