Buffy and Dawn thinking Tara being dead is Willow's fault feels extremely OOC.

If anything, the characters would blame Buffy for not dealing with Warren before he could hurt anyone else. Tara was the innocent victim of a stray shot in an attack on Buffy. The canon continuation of the series in comic form even has Willow feeling guilty for subconsciously blaming Buffy for Tara's death.

There's many legitimate ways to make Willow look unsympathetic after what she did in season 6, but having the characters blame her for Tara's death feels like a bad choice and and out of character.

Sorry, it's just that moment in the story broke my "suspension of disbelief".