So sorry to hear you are going through it too RaspberryRuffless and interested to hear you have occasionally used the same approach.
Hissy I actually received the email whilst ex was here and showed him. He said he had no idea she was planning to email and that's how we left the conversation. They are now away on holiday for 10 days so they have time to discuss things further.
I used the term separation anxiety for ease, but dd's fears around my safety are more complex. It is more like a phobic reaction or an extreme OCD ritual. She believes that if I leave the house without her, I will be in extreme danger i.e. at risk of dying. She can keep me safe in 2 ways 1) by being with me or 2) making sure I remain in our home if she goes out. So, in lots of ways she is making progress - she is able to leave the house without me, both on her own and with others but on the strict proviso that I am at home at all times.
We have been through many attempts to ease her over this anxiety by tiny steps towards me leaving the house briefly etc etc. Instead of this building her confidence, it seems to reinforce her fear - almost as if by going out briefly and coming back safely she feels I have 'dodged a bullet'. Rather than reassure her that there was no extreme danger in the first place, she feels that the odds are then even greater that I will not be so lucky next time!
Some well meaning friends/family have advocated a 'cruel to be kind' approach, with the expectation that if she is 'forced' to experience me leaving and coming back safely she will learn that I can be safe away from home. I have explained that this would be akin to forcing someone with claustrophobia into a box and locking it, to 'ease' their fear of enclosed spaces.
But, as I have explained, we are fully engaged with professionals so hopefully one day we will find the therapeutic approach that works for her.
Sleepovers have always been extremely difficult for DD, and she has only ever stayed with exp (never at friends or other family without me). But even with exP she found it extremely stressful and barely slept. As her anxiety increased and mental health deteriorated, this became too much for her and she hasn't slept at their house for several years.
ExP being here that night was the closest to a night off I've had in a very long time. I was able to go to bed and know that however long it took DD to settle, he would be there for her, and if she woke, she could go to him not me.
It seemed to be a win win win - DD slept brilliantly, exP got to be a full on dad for the night and I got some respite in the haven of my own room.