I expect that McClellan will be on the team in the next few days, with Hightower sent to IR.
Mitchell could be signed instead of Valentine.
I believe that Mitchell is unlikely since he hasn't participated in anything for a long time while Valentine has been participating. Yes, I fear that bringing back Mitchell early could hurt his career. I do see him as injury-prone, and I think he could have a fine future for us. He's already shown a lot.
IMHO, we need Valentine. However, I do agree with your inference. It is indeed possible that Valentine isn't physically ready in which case Belichick may keep him out until next year. In this case, we would use the Practice Squad or trade block for a backup DT.