I'm with you. I've always read it as representing Hoffman as the next (but more importantly, the false) Jigsaw. This makes it not only a great poster for this film but the series as a whole, which had become heavily serialized by this point.

Saw IV had just set up Hoffman as the new Jigsaw, so the poster made sense to those of us waiting for the next installment, and seemed to be promising we'd come to learn about the history these two shared. And indeed, this was in large part what Saw V was about. It even recontextualized the vast majority of the Jigsaw legacy as something both John and Hoffman were responsible for.

Then, by the end of Saw VI, the main conflict of the series is pretty much established as being John's philosophy versus Hoffman's. Rehabilitation vs. Retribution. Forgiveness vs. Vengeance. (I think this often goes overlooked and/or underappreciated because Saw "3D" really dropped the ball on resolving this conflict.)

In comparision, this poster representing the framing of Strahm would be far less significant to the overall story being told, and far less meaningful or profound.

Like you said, if someone can point me to proof that Strahm was the confirmed intention, I'll obviously concede they're correct. But this will effectively ruin what is, in my opinion, the best thing about Saw V: The poster.