Why does Bitsey Bloom think that a full length murder tape exists?

The tape found in their motel wasn't proving anything and it could be even a fake. But Bitsey starts believing that David is innocent and that the real murderer is toying with them all hours before the execution:

Bitsey: [The murderer] knows News Magazine won't give the scoop to the dailies or nightly news, not after having paid for it. Probably guessed we wouldn't call the cops. And, most importantly, knows I'll tell Gale tomorrow.[...] What if Harraway's murder was just a means to getting Gale. I mean sending him through hell, a sick cat and mouse game. You kill the one person he has left. [...] You destroy his loved ones, his life's work, his memory -- and you make him watch.

So at this stage, all this tape proves to her is that David has been framed - she doesn't think yet that there might be a full-length tape. That is until she goes with the experiment trying to recreate alone the circumstances of Constance's death. Then she realises that it was a suicide:

Bitsey: [Constance] lived for DeathWatch. If she's gonna die, why not die for it? That's why the tripod was here. To record proof, undeniable proof, the tape. That's why we got an out-take. [...] She needed help, someone to keep it, release it. Someone she could trust, someone dedicated to the cause...

So from this moment she believes, that Constance and Dusty have orchestrated the "murder" to frame someone, just to put that person on death row and then pull the tape to show how justice can be perverted and sentence the innocent people to die. But Bitsey thinks, that killing David wasn't part of the original Constance's plan and Dusty (portrayed as an extremist) has decided to sacrifice him to drive the point harder. This would mean that Dusty must have the tape that Constance was planning to send to the judge shortly after announcing the death sentence.