If i accidentally save and quit and rejoin after only 2 seconds because i missclick in the pause menu, is the run invalid?

no but its harder to verify

These things happen. I assume that as long as the video is clear and there is no reason to suspect anything, then you should be fine. Maybe if it was a world record run there would be something to say, or something to look into, but if you accidently clicked main menu on your 30 minute run and rejoined the game, then you should be fine.