I would feel more comfortable drawing conclusions about the letter and any intended meeting if we knew the circumstances of the writing. I don't believe she wrote the letter without the police knowing about it and being fine with it. Maybe the first one, but if they were unhappy with her writing the first, they would have made it clear not to write another.
That leaves two possibilities. First, that she wrote it completely on her own and the police stayed out of it but were ok with it happening. If this is the case I think there is very little we can infer from the substance of the letter. She might think she knows but is wrong. She might know it's someone in her family but that the rumor mill is looking elsewhere so she wants to continue the push in that direction. Heck, she could have read stuff on here and drew conclusions based on what people think here.
But I think it more likely that LE helped draft the letter. I know several have said that the wording sounds hinky. I think it sounds like many speeches and letters written for family members by LE to help rattle the perp. Sometimes you hear family pleas at a vigil or an anniversary of a unsolved murder and they were written at least in part by LE to target the killer and ratchet up the stress in their life. In the letter she is pointing out what is going to happen to the perp. He/she is going to be cuffed and hauled away in from of the person's family. The thought of that has to be near heart attack inducing. I think the comment about the boots is telling as well. If the police helped her write the letter, then I think they believe the boots were at least a bit too big. The perp probably thinks that was a smart move to help mess with the identification. But pointing out that they know and weren't fooled will also cause stress. I also think the fact that it was written without reference to gender is a real sign of police assistance. That is very difficult to do. Even in these posts I struggle to reword and remember not to refer to 'he' or 'she', and I'm not fully in one camp or the other yet. I think the police helped her be sure not to give away the gender of the suspect.
I don't think the police expect the letter to cause the perp to turn him/herself in. But I think they hope continued stress and pressure on the perp is going to make it more and more difficult to act normally.