As for the butt height thing, that largely has to do with the Center stance more so than his height (though their height can play a part as well). I mean I've seen short guys with their natural stance with their butts high in the air and I've seen tall lineman in a crouch like stance with their butt real low.
As for the swing 6th lineman, I keep reading that it's open for grabs because of the Tretter injury...
I'm not sure why so many thought Tretter had that locked up. The OL coach said he's never seen nor knows if Tretter can play center so no reason to think he had that swinging 6th linemen job over
Bakhtiari (who was drafted higher)
Barclay (who looked alright last year)
and others.
As for as who can be that back-up Center person, they're been having Barclay practice at RT, RG and C... sounds like ever so often he's having troubles with the snaps though so he's probably not ready for Center. Though MM said Lang can play Center so maybe he could be Center back-up and the best back-up to OG can be the swing guy.
Also you got Van Roten who was called up shortly after the season started.
And some UDFAs from this year...