My argument is based flatly off watching what happened. I'm going to be that guy, but sometimes you really do have to watch the games. I know you're all in on plus/minus and really don't like Westbrook, but I'm running in circles trying to explain at this point. Every time I look at RAPM I'm finding that we're ultimately looking at a game and taking everything we want to see as fans and projecting it onto certain players, and I'm finding it bizarre that certain traits inherently have made players "good" or "bad". The board has suddenly proclaimed Davis better than Durant and Kawhi is a top 5 player, on the basis of a single metric.
In the end, Westbrook had a team in which the following happened, and I'd note all of this is affecting the numbers you're seeing:
1. Start the season, roster is 8 men deep.
2. He returns. Looks okay.
3. KD comes back, fantastic play for a few weeks, he goes out.
4. He looks fantastic for 2 months.
5. Adams, Roberson, and Collison go out while Perkins is traded for Kanter. The defense of the entire team is shot, completely wrecking any semblance of ability to use plus/minus effectively for defense.
6. Adams comes back, team looks amazing again.
7. Ibaka goes out, defense again shot.
So, in summary, Davis's team had actually more continuity w/o him than WB's. There's nothing plus/minus can tell me when the team missed this many key members for weeks on end and they never had a full lineup over 2 weeks. I'm all for the data, but in the end if I can't get any complete data, I need to use what happened and look at what would likely be inferred. Here's an image for games lost and impact (PO teams are green):

Again, I realize you can't stand WB's game, but he had nothing to work with. Given the lack of continuity entirely and what he did, and the fact that according to everyone outside OKC, his coach was as incompetent or moreso than AD's, people just aren't giving credit.
And one final number, to emphasize....
Team games in which players were inactive:
OKC: 196
NOP: 98
Plus/minus can't work with that small a sample and be reliable, its like using it for the playoffs or other small samples early in the season.
I needed a rant. More coffee please this morning.

Edit (re-edited to 2 man lineups):
Lineup minutes for the most used 2 man lineups:
AD:
1915
1375
1320
1110
878
Westbrook:
1411
1228
998
980
778