Sure, Rak can play C. As a stretch 4, he can slide over. I certainly like him better moving over than the other forwards because of his strength an athleticism. It's not like he sucks. But once our taller/beefier 5's are functioning properly in there, Rak will be assuming his natural position at PF.
Unfortunately, as you point out, he has a forward's body and a 5's mentality. That works in the fall against mid-majors, but later on, when we play conference or tournament teams with legit centers ... he's overmatched. The two kids from Seton Hall pushed him all over the lane. Adams (a freshman) pushed him around. He played cooley almost even in that one game you mentioned, but got killed on the boards last time we played ND. Obviously, he was overmatched against Michigan.
At forward, I agree with you (and said above) that Grant is able competition. He's probably more productive offensively, while Rak develops his shot and handle. But we're better ... and Rak's better ... when he plays along side a center instead of pretending to be one. He'll start at PF and JB will pray once again that the light goes on. If he doesn't produce, Grant will eat up his minutes. Or he can play spot duty in the middle against smaller teams ... but as DC2 comes up to speed and with Keita playing very well, Rak had better put his PF cap on if he wants PT. It's big boy time.