I dont think the UFC will create an interim title, although i may be wrong. More than likely they will give Mir the winner of Nog/Velasquez until Lesnar can fight Carwin.
If this fight did happen, though, Mir would probably win. Carwin is a bit overrated IMO and would be finished immediately by Mir's jiu jitsu if the fight went to the ground. Carwin has had a nice run in the UFC and packs a very powerful punch, but Mir would put him in his place if they fought.