I wouldn't agree Mertesacker is bad in the air; he's often like a magnet in the box, makes so many headed clearances. Far superior to Koscielny aerially for sure.
I'm all for Marquinhos if we can get him, but I think a partnership of him and Koscielny next season would be pretty disastrous. Maybe I can agree with what you've said recently about us not necessarily needing a shouting, pointing leader back there, but we need some experience and intelligence at the very least because Koscielny doesn't give anything beyond his own performance and I doubt Marquinhos would either.