You make a very good point, of course. Consider though, that Misty has not come clean and has never admitted to not being there. On the contrary, she has always insisted she was there the entire evening/night.
Whether anyone would believe it or not, or it could be proven or not, really is no reason for Misty to not admit it. She has said a lot worse that could not be proven, IMO. The fact is, she has not admitted this and I think she would have since it would be an admission that would not point to her as guilty for Haleigh. Unless LE has said to Misty directly that they know she did not harm Haleigh, she must think that they suspect she did. Misty may not have much to gain by admitting to not being there, but she would have nothing to lose at this point by the admission, IMO.
IDK, these people do not think normally. I do not believe Misty is protecting Ronald or Tommy. I thought she was for awhile but I do not see how she can be strong enough to take that protection this far if she herself is innocent.
JMO.