C.T.
Hi MC,
That is a lot to follow but my gut reaction is stay neutral because those children desperately need a neutral party. They deserve someone to remove themselves from the drama and be an island of sanity. They need to know NOW and many years from now that their love-ability is not contingent upon who's side they felt they picked and they will feel this pressure. Your ability to stay mature, focused on the children only is all they have because their parents cannot.
I would not write a letter on his behalf but write a letter to the COURT about your love and devotion to these children and your hope that the parents can obtain counseling so that they can step up in their respective roles and stop arguing..
You will take heat no matter what, expect it and even expect the kids to maybe feel compelled to neglect you now but your presence and honesty will surpass all of this.