Seems too old too

He was a terrible choice! He was way too old and way to fat.
This must not have been important to the Directors which was a big mistake as it makes the film very hard to follow.

I'm not sure if that is the point, as almost every time they got a new actor to play someone they were totally different in looks and in vibe (and often in height, too) to the people who'd played them before the two Paulines were nothing alike, adult Anton was nothing like his teenage or early 20s self, Hermann was completely different every time you saw him, etc. It seems to me that casting someone similar to the actor who'd already played a character just wasn't at all important to the casting directors.
I also agree with ianevs79 who said he looked too old he looked a lot older than his contemporaries such as Eduard, Maria, Glasisch, Pauline, etc. who were all their old selves (except Pauline, but she'd been playing Pauline since Pauline was in her 30s) just with aging makeup on. It didn't matter so much after episode 8 when everyone was made to look a lot older, but when he first returned in 1947 he looked a good ten years older than everyone else his age.
"I only ask that you respect my jumper!"

He doesn't even speak German properly!

That is the point, he is a completely different person when he returns. His manner, his Americanism, the old Paul doesn't exist anymore.