Yeah, meeting the clear terms of ritual magic seems like quite a freakish coincidence unless that had been their plan. So I think they tried this, or at least tried to give the appearance to Voldemort of doing this, so that he would feel himself safe from other traps.

As for the scar, anyone involved could have given Harry the scar, in a myriad of ways.

Ha! Maybe Dumbledore gave it to Harry after finding him alive in his crib.

Evidence - unidentifiable burnt out husk of a body. Scar on Harry. Voldemort at the scene of the crime. Voldemort presumed to want Harry dead. That's pretty weak tea.

The best bit of evidence I can see is the one no one talks about - the feeling of Doom and magic resonance between Harry and Quirrell. It seems credible to me that Voldemort might not have anticipated that, and it was Dumbledore so booby trapped Harry with that resonance.