Other than a few egregious examples, the games aren't fixed, but the league can cleary skew things by how they direct officials to call certain plays.
Two examples were I think things were fixed were Kings-Lakers 2002 Game 6 (no question) and Bucks-Celtics Game 7, 1987 (Pressey picks up 3 fouls in a short time span and fouls out to free up Bird).
Harden/Irving and Middleton/Holiday are going to be important, but I don't think those guys are going to be the key to the series. It really comes down to Durant/Giannis and, even more so, how they officiate Giannis on both ends of the court.
There is probably no player that ever lived better equipped to guard Durant one on one than Giannis. Even most elite defenders like Draymond Green (when Durant was at OKC) or Kawhi Leonard usually lacked the length to challenge Durant's jump shot. Durant was going to get off a decent shot against them and there wasn't much you could do about it in single coverage and that forces a double. The doubling was lethal against the Warriors with Curry and Klay to take advantage (but not with OKC with Westbrook clanking shots). Giannis, on the other hand, has the length to bother Durant's jump shots while still having the lateral movement to stay in front of him.
There are two keys. One question will be whether the officials allow Giannis to play physical defense against Durant to avoid foul trouble. A couple of soft foul calls on hand checks or challenges will completely change the nature of the game. Needless to say, the block/charges on the other end when Giannis is involved will have the same effect. The Nets have no one to guard Giannis, so expect a lot of Blake Griffin/Harden flops on their end.
If the game is officiated to allow Giannis to stay on the court (and he avoids careless fouls), the Bucks win in 6 or 7. If Giannis gets in foul trouble against Durant consistently, expect the Nets to win in 5. My guess would be that the officiating will differ depending on who is the home team.
I am suspicious that with almost all the big-market and/or coastal teams other than the Nets perhaps being eliminated soon (Lakers, Clippers, Knicks, Celtics, Wizards, Heat) the home office may go out of their way to skew things towards getting the Nets to the Finals.
Phoenix/Denver/Utah/Dallas coming out to the West really hurts ratings. ESPN/TNT would obviously want the Nets coming out of the East. The Bucks or Sixers would at least generate some moderate interest. The nightmare scenario would be the Hawks coming out of the East to play one of those 4 teams I mentioned. Almost certainly not going to happen.