If Rocco suspected that he was being used by Michael in the Roth assassination attempt as a way of evening the score for his participation in the attempt on Michael, wouldn't he have run the other way?

I agree with Turnbull here. Rocco's designation of himself as the Roth assassin was his way of trying to regain his former position in the family, which had seemingly been usurped by Neri.

Besides, if Michael suspected Rocco he wouldn't have found it necessary to contrive the elaborate scenario in which Rocco kills Roth. He would have simply had him whacked the moment he was convinced of his suspicions.

Suppose Rocco had been taken alive (which is a whole 'nother question - why the guys guarding Roth shot and killed Rocco, rather than try and capture him)?