Yes, but that's why I think Michael was 95% certain immediately that Frankie wasn't the perp. Frankie, as you (and Michael) put it, didn't have the wit to pull off the shooting. And what would he gain by it? Michael knew that Frankie didn't have the head for the big deals that might have presented themselves if Michael were dead. So, Michael figured, Frankie had little to gain by whacking him, knowing that he'd set off an orgy of vengeance and succession-grabbing by Rocco, Neri, Fredo--and probably the Rosatos and Roth--that would soon have him in the grave.

But, given Michael's suspicious and controlling nature, he had to be 100% certain, which is why I believe his outburst was a test.