does the last hit on the boss count as the end of the run if the character died at the same time as the boss?

I think that if the last hit triggers ending cutscene or credits it counts, like in cnd1 or Darkwing Duck. I'm not sure how that works in this game.

I would see this as even if the last hit is delivered you would still need to have proprely beaten the game for the run to count. Not a mod but thats my opinion on the topic.

No, in this game the cut scene does not start, the level starts over

Once again only opinion but i wouldnt consider the run valid since the game isnt beaten.

In principle, I think this is true, the general rules of the speedrun still imply the passage of the game completely, it’s just interesting to know the opinion of the runners

It's a pretty good question (sorry for being late on the topic). if the Double KO would lead to Ending Credit, I would say yes. But the game is still on so the game is not beaten