exhaust manifold nuts

[goingcrazy]

I'm working on an 03 Vibe base and it needs new motor mounts and the exhaust manifold replaced. I got 3 of the mounts swapped but the rear mount is an issue. Clearly the extremely long stud is an issue so I thought I would remove the exhaust manifold to get more room since I'm replacing the manifold anyway. here is where I ran into an issue:

I have a fairly large selection of metric sockets but it would seem I don't have the right size. Nut holding the manifold on appears to be a 10mm but my 10mm 6 point doesn't fit (too loose) and the 9 and 11 are the wrong size. does it require a 10mm 12 point or am I missing something here?

will removing the manifold allow the rear mount to come out or do I need to remove the cat as well?