You actually touch on a philosophical debate that advanced stats experts have on assists. The PPR (pure point rating) is a statistical formula to determine a player's playmaking ability. In it, a turnover is more costly than an assist is helpful. The thinking is that you don't need an assist to score points, and that the assist does not actually directly create the points either, so you can't say "hey an assist, it was worth two points" when he didn't actually make the shot, he just passed to a guy who did. It isn't directly worth 2, just like a turnover isn't directly worth 2 points in the other direction.
If you want to talk about how much he contributed, look at his GameScore stat or +/- stats, and both are ty. In fact, in a thread a couple months ago we had looked up his GameScore and +/- stats for the playoffs and they kept going down as the playoffs went on. Manu got most of his stats in the first 2 series and barely did since.
Onto your "his assists got us this and his turnovers cost us this: Actually his 22 turnovers also cost us at least 22 points on top of those 26, so that's 48 at least. And as I said earlier, Manu should not get full credit for the points scored on an assist, you're really trying to stretch your dollar with that. At max he should get half credit since he's not even taking the shot himself, he's just half of the play. That's what advanced stats are meant to be for. So we'll say he's responsible for adding 36 points making plays. That means he still cost us -12 in net total, so his playmaking was a negative, not a positive. That's on top of his poor shooting outside of games 5 and 7. Overall, Manu barely had a positive impact on the series besides being a decoy outside of games 5 and 7. This is reflected in his GameScore stats, which were right around ZERO for most of those games. 10 being "average".
And btw, feel free to use the same logic on Tony Parker's performance if you want to try bashing him. I use the same logic when determining the usefulness of any player's playmaking, this isn't something I cooked up just for this. That's why I defend Parker's performance and bash Manu's. Parker had a positive overall impact despite the leg injury, Manu did not outside of two games. And one of the two (game 7) is basically what we typically expected as recently as the season before. Minus the choking at the end.
His shooting numbers were as bad as they seemed, I posted his shooting stats already including and not including his 2 good games. Games 5 heavily inflated his overall %, but I prefer looking at the mode rather than the average. I'm not arguing Manu helped us win game 5, I'm saying he was really bad in games 1-4 and game 6 especially, so game 5 and 7 don't matter.
As for how he did against OKC. . .honestly, if you look at that series too, he SUCKED at OKC. Like a huge holy level dropoff. He let James Harden beat the brakes off him all series too. Though I'd happily take a "50/50" good/poor Manu over a 71% chance of below average starting NBA SG impact (14% of which is a HUGE negative impact, and 29% is a well below average, borderline neagtive impact), 14% chance of an average game, and 14% chance of a good game that helps us win it.
That's what it boils down to. He was poor in most of the games when he's supposed to be one of our better players. I couldn't give a less if Leonard missed afree throw. Manu missed his free throw right before Leonard did and also sucked hard that game while underperforming throughout the series while Leonard was pretty good throughout the series and is already known to not be a great FT shooter. That's false equivalency. Manu was far worse than Leonard from expected impact and actual impact standpoints.
Manu's entire season was worse than expected, and his impact per minute went down really far in the finals. Dude played 20% more minutes and still put up less numbers. So he was about 20% worse than what you should have expected from his already below expectations season too.