I think Locke in the centres would work well.. in the sim at least. If you're going to do it, though, I strongly recommend playing him on the same side as Mannering.
Mannering to help him on the defensive side of things obviously. EDIT: That will also help out Ukuma, who doesn't seem to be much of a defender. Having Tate outside him will help the bugger out.
Indeed. If you have three iffy defenders together on the same side of the field as far as half, second row, centre goes, it can get ugly. Locke would be fine if Mannering was there though.
I've got a feeling Lillyman could be good in this if played at second row. He is a great defender one on one and is pretty fit from what I've seen so he'd have no problem covering centre.