Others may know much more than me, but I haven't seen anything that gives me the impression Brittney was anything but close and loyal to her family, and loved them. For better or worse.
I just can't see her agreeing to testify against them, or taking witness protection. I read somewhere that the reason witness protection programmes sometimes fail is nearly always because the witness goes back to see family or friends. I think that's exactly what Brittney would have done. And I can't see her agreeing to accept she would never be able to see her daughter again.
And to cut a hugely expensive deal like that, what must she have known? LE have been able to bring serious charges against family members without her help, so I doubt it was that. It doesn't seem it could have been major drug running either - aren't those people usually rich?
I really don't think Brittney is safely ensconced in a witness protection programme, but I'm still unsure if she did know something about somebody that made them think she had to disappear. Or if she discovered something, or was asked to do something, that caused her to come out fighting. And lose her life in the process. This'll probably sound completely crazy to you all, but what about if Brittney was still around when DH committed suicide, and didn't think it was suicide at all? If she was loyal to him, what might she have done, or threatened to do?