U, I am not surprised that they will be used as leverage in negotiation. What I would be asking is the lawyers opinion on what they can get by using it as leverage (is it 45-55 or 40-60 etc for custody instead of 50-50). Also, what steps can you take to prove to a court that you are a good father to your kids? Documenting your time being spent with the kids over the past six months is one of them but there are other things you could probably do.
Just my opinion but when you talk to the lawyer, you probably want them to understand your entire sitch and give you their opinion on what will happen during the D. Make sure they can account for all things and not just the letters.
In one of your previous posts a while ago, you had mentioned you talked to a lawyer and they said the letters were not an issue. Curious what changed now? Is it because the lawyer did not read the actual letter at that time?