Is it possible to actually invade pewdiepie's minecraft seed world and maybe run into him when he is doing his minecraft stream?

Maybe…

I doubt he actually plays himself… But probably!

The seed is used to generate the world. Are you stupid?

No. Having a seed doesn;t mean you have access to his server. It just means you have the seed to generate the same world that he is using. But his game and his save files and all of that are on a private server you need permission to enter.
And thats just for when he has other people on.
When he's just playing alone, he's likely playing offline in single player mode. So nobody can join in.