I believe that if MG truly had more to do with this case, she'd already be in custody. Being so close with Lori and Chad, there would surely be more of a concrete digital/paper trail associated with her than any of the other "players" in this tragic story, especially considering their history predates when all of this super crazy nonsense got out of control. Considering she is not in custody, and that she also did agree to be interviewed for Dateline, I don't think MG is hiding anything further.
What she told police (which we are now privy to thanks to Dateline), if true, is invaluable information - not only in helping confirm the dates and context of JJ's last whereabouts, but of the headspace of Lori and Chad at crucial times, clues to possible motives, and further knowledge of their "religious" beliefs. All of which MG apparently willingly and voluntarily told police without any pressure from LE, done entirely of her own accord.
I'll also add that I have no idea how I would act if I were interviewed on national television about my former friends turning out to be murderers. Given the knowledge she had and what she witnessed, I wouldn't be surprised if the reason she was stoic and didn't shed a tear is because she has long suspected (as we all have) that the children were in fact deceased. She probably knew in her heart the moment Lori called her to ask her to lie. Moreso and much earlier than the rest of us.
Unless we find out anything new, I don't really suspect MG of anything other than being an extremely poor judge of character and gullible to conspiracies. She had more sense than anyone in that group at least.