SadSam, I so relate to what you're saying .... are you sure you're not me ??!!
My DP & his ex have been separated now for more than 6 years, yet the sense of bitterness, hatred, spite, malevolence, lying, emotional blackmail, hysterics etc. (not to mention using the skids as weapons) seems to progressively get worse, not better, with time, as you might expect. Theirs was an agreed separation - no affairs, no alcohol, no violence or anything else which you might just be able to understand leading to lasting bitterness. Within weeks of separation, ex was out with new b/f (a "friend" from work) and introducing him to skids. She's had several b/f's since then, including, the married father of her kids' friends ! However, when me & DP got together more than 2 years after the split, WW3 broke out ("how dare you introduce that woman to my(!!) children" was the least of it) ....
.... seems to be a classic case of "having her cake and eating it". Despite having a b/f now, she seems consumed by bitterness and determination to ruin the skids' relationship with their dad - and if she can cause trouble between me and DP at the same time, that will be a bonus. I've said before that I feel haunted by DP's ex .... some of the stuff she says, though usually it is assumptive nonsense, is just so personal and way beyond the boundaries of what an ex should be remarking upon (so far as I'm concerned her only topic of conversation with DP should be the skids, nothing else).
Like you, I just can't get my head around it .... DP & I have spent what probably amounts to days going round and round in circles about her bizarre and damaging behaviour but getting nowhere obviously as we cannot see inside her head. However, we suspect from everything she's said that it is a case of very extreme envy because DP has moved on, got engaged, bought new house with new partner, had another child .... in other words he seems pretty settled. But, of course, there's nothing stopping her doing any of this if she wants to and circumstances allow, and DP wouldn't dream of commenting on how she organises her life now. You know, it seems incredibly childish, but all her tantrum like actions almost seem to point to a fury that DP did it "first" .... like, "how dare he?"
"Your" ex sounds like "ours" .... she doesn't want him but doesn't want anyone else to have him either. "Our" ex wanted the freedom of being a single woman, to come and go as she pleased, see who she wanted etc., but also all the advantages of being married, i.e. in her eyes, to have DP still at her beck and call and still to be the most important woman in his life even if they were separated. This may sound fanciful and even perhaps paranoid but without citing numerous examples of what she's said, I can assure you that those comments I've just made are based on stuff she's actually said. Ex is a very very selfish woman and I think she thought she could "have it all" for as long as it suited her ..... when the reality of separation & divorce hit home, she didn't like it one bit .... hence 6+ years on, and we feel like we live in a war zone.
Heaven knows what the answer is ... we used to believe that time would be the healer, that, and new b/f (s) but goodness knows how long we will have to wait as those factors don't seem to be working. I am scared now that this is how it's going to be until youngest skid is 18 (in 10 years time) and she can no longer use them quite as effectively as weapons.
I really really hope you get somewhere with the letter you've written. But if she doesn't agree to be fair and reasonable, I still think it very unfair of your DP to jump to her demands as and when without discussing it with you. Maybe ex might get a bit of a wakeup call if your DP tells her he won't be dictated to .... e.g. he tells her that suggested dates are NOT convenient, but suggests his own instead.
Good Luck ... and sorry for hijacking your post a bit with my rant against BM.