their flimsy cover plan

They don't mention framing anyone as Sam's partner.

But she'd need to have one, because it pretty quickly became public knowledge that Sam, Tara, and Richie left Dewey alive before he was killed.

I guess the best candidate is Tara, with them being sisters and so much revolving around them. Because they think the twins are dead, and obviously it wasn't Sidney or Gale, and that's everyone left. But Tara still wouldn't explain Dewey's murder. Unless Richie would've retconned it into making it have been that Dewey just stayed to wait with the deputy and Sam went back to kill him or something.