Anyone know what will happen to returning explorers when there's already a 10 dweller line outside your vault?

Nothing he will be standing as others in a line.

Ok, good to know thanks. So I guess the 10 dweller limit is only for newcomers.

I wonder if there's a cap for total no. of dwellers then. If I keep playing the way I am currently I might have many dozens of dwellers running around in the wastes in the near future. Could get very crowded in front of the door when lots of them return within a short time frame.

200 max cap of dwellers at the moment

No, I mean total no. of dwellers that can stand in line in front of the entrance, not the total vault dweller cap. I am already at 200.

Space for lining up is limited, so I wonder what happens if there are more explorers returning to the vault than can be shown on the screen.

You can have 10 waiting who are NOT members of your vault. As for your explorers, they will join the line but do not count towards the 10 limit. As for having a line of explorers longer that can be shown on the screen, I have no idea since I have never had more than five or six out at a time. Just a wild guess, I would say that if they are not on screen, it will show them as returning with less than a minutes left and they will remain in that state until there is room for them on screen.