You're playing up how the bowcaster is so powerful that its shot sends people flying, but not addressing the fact that Kylo doesn't go flying when he's hit. That suggests to me that he doesn't receive nearly the full power of the blast, and the location that it hits provides a very plausible reason why. You haven't given a plausible reason yet why he suffered the full impact of the shot and didn't go flying or even get knocked back.
No, I wouldn't have said that if I just didn't like your explanation. Give me more credit than that. You really didn't explain your argument well, even though may've thought that you did. You did much better this second time, so I now know what you were getting at and how to answer.
The reason is that he's not an ordinary human. He's a trained fighter and Force master, capable of incredible feats, both physical and mental (no doubt including blocking out pain). He shouldn't be so incapacitated by a few setbacks that he loses to an amateur. Can you see that happening to Obi-Wan or Yoda? Even if they were missing limbs, they would likely be able to prevent anyone but another Force master from even approaching them. If Kylo were an ordinary, untrained person, yes, it'd be easy to believe that a few injuries and a lack of focus would explain losing to another ordinary, untrained person, but he's not one. That's your answer for why it's hard for some of us to believe.
I'll bite again.
1. I'm assuming JJ believes that because Kylo is a Force user his body could handle the impact of the shot better. What's the alternative? He gets shot off the walkway to his death? He wrote himself into a corner but that doesn't change that the movie took special care to demonstrate the power of that weapon for a reason. It's a borderline Chekov gun device. And as though all that wasn't enough, they took special care to show he was still bleeding.
Chewie shot him from a balcony, not a mile away. You shoot someone with a regular gun from that kind of distance and it's going to do considerable damage.
It doesn't matter that he didn't take the full impact that others in the film had because the narrative distance between "not being sent flying" and "getting grazed" is pretty large.
2. pass
3. Yoda had hundreds of years of experience while Obi Wan had a couple of decades. That's not a fair comparison, but that's also another flaw of the prequels. Save for fighting Dooku and the Emperor, Obi Wan and Yoda respectively were shown to be virutally unstoppable machines whereas other jedi/sith counterparts weren't. Even a minimally untrained Luke got a slash on the shoulder of Vader in Empire.
But past that, again, Kylo in the moment that Rey turned the fight on him was unfocused in the erroneous belief that he had gotten Rey thinking about joining him. You keep saying I didn't explain why it's possible that he has a trained fighter got thrown off in that moment, but all the character evidence you need is in the fact that Kylo was no longer trying to kill Rey in that moment. All he had to do in the moment that Rey closed her eyes is kick her off the ledge, or break the saber lock to slash at her or stab her, or go for the head. Any number of things in that moment that a trained fighter would recognize as being handed a victory on a silver platter. There's a character reason that he doesn't. He got thrown off by Rey's pushback and when they lock each other by the wrists she got lucky again and gave him a third injury to his foot allowing her a second or two to slash at his face. It matters that he's injured because he wasn't at 100% thus making it easier for Rey to throw him off guard.
And that's also why it's important to mention that Kylo was dominating the fight for a majority of it. He relented when he tried to get her to side with him, went soft and Rey blitzed him. It was a hail Mary move that worked in her favor but it was hardly a fight where the underdog was impossibly on even footing with the better adversary the way say, the USA team was with the Soviets in 1980.
Like the way I interpret the scene is that in any fair fight where Rey gets no handicaps in her favor, Kylo would've won 10/10. And to me the conditions present were still tough for Rey, but for me plenty believable. But I don't look at the scene as though Kylo got a fair fight/remained focused the entire time.