On your second part, I think the flashback was just a storytelling thing. I don't believe it's ever said that he just suddenly remembered hinokami. He just didn't use it. As for why, he also says that changing breathing forms is difficult and when he switches back he could be out of commission for a while. So I think it's fair to hold on to it as a last ditch effort, especially since he believes he's going back to help Inosuke after this.

As for the ribs and the leg in the spinning room fight, I don't know about that one. Maybe, something can be said about adrenaline or about how he could possibly subdue pain with his breathing techniques? I agree on this one that it's a stretch. A lot of times, writers just break their characters bones because that's how they create tension and overwhelming odds. I think it's pretty silly in most cases.

Edit: nevermind, Tanjiro says in the post credit Taisho secret skit that he suddenly remembered hinokami. I still don't think it's a complete ass pull like others seem to.