Hi HPDhelp:
I agree with Caro. There are 2 potential things that for me point to HPD. One is the tone of total calmness that the letter is written in. What she is basically saying is an "eye for an eye" a "tooth for a tooth". She is also saying "don't throw stones if you live in a glass house"...and you do! You potentially have your job, your wife, your daughter and your home to lose...and trust me...that is big time serious (though you may not realise it yet...)...

! For me, her last phrase is the second point and is so, so, so HPD!! However, what can you do about that? Absolutely nothing. By trying actively to turn her family and friends against her you shall just turn them against you and increase their protection towards her (as a needy, selfish little drama queen, its probably exactly what she wants to obtain from the situation whilst ruining you; double win for her, double lose for you...)!
My recommendation is that you now keep away from her friends and family, and concentrate upon your own. Your reactions are human, but based upon what she says, she is ready to slay you and already has enough evidence to do it. Don't give her anymore, and to calm things, you may wish to write her an appology letter (you are not really appologizing, its a strategy to protect yourself and undermine her actions against you...) explaining that you were very hurt and therefore you lost your temper; that you wish her well, that you are trying to forgive and forget; and that all this won't happen again. Even though it may go against what you feel, you must calm things and smooth it over. You shall need to move forward with your head not your heart.
After this I recommend you forget this girl as soon as possible, and if you cannot, I recommend that you get some therapy to help you distance yourself before something really dramatic happens. I too was (and still am...) married, have a little girl, and a home to protect, but I was the one to break it off with my X, and then having talked it through with my wife, we stopped our divorce. Today i am very much in love with my wife again, I receive much love in return, and my whole family and home environment is worth gold. Unfortunately you sometimes have to almost lose it to realise that. I have been given a second chance. However, I never once tried to contact my X's family (I think they at least know very well with whom they are dealing...), or her friends about how she is, which is my perception, and the perception of numerous others, but without doubt not everyones. Also once I ended the relationship, my total contact over 4 years resumes to 4 or 5 emails when I decided i want to never have have anything to do with her anymore (we had stayed close friends for over a year after i broke off, but she was constantly enticing me to her which I found perverse as she knew i was trying again with my wife; she was encouraging me to work at my marriage on the one side; yet perversely, trying to get me back into her bed on the other...paradoxes and confusion!!), and the 2 recently where i had learned enough to be able to confront her factually and coldly on what i perceive ot be her absurd actions during our relationship. Therefore, my situation may be somewhat different to yours.
Please take care HPDhelp I think you may be (sorry if i am off target) walking a very thin line between indifference (the desired and only positive direction for you...) versus passion and hate (this could go very bad...). You need to make sure you fall on the right side of that line.
Musician