If he comes back & if he uses the funeral as an excuse for a row, I think that'll probably show him up once and for all as the uncaring idiot that he is.
It hardly needs spelling out does it, that a funeral occurs once - there are always other birthdays, indeed, many many people hold birthday celebrations on a different date to their actual birthday - due to work committments etc ....... Jeez ......
Your DP comes across to me, from what you've said (and just confirmed again in your last remark) that anything which even slightly disrupts the skids' arrangements, is a terrible slight upon them - and maybe him too. That's a ridiculous and unrealistic attitude, because, in real life, things do happen unexpectedly every so often and sometimes, just sometimes, things happen which are, right at that moment in time, more important than the skids. This isn't me being horrible, just saying it like it is - or should be .... apart from anything, children too, even relatively young ones, shouldn't be totally cocooned from life's realities and at some stage, have to learn that on very rare occasions, other people's feelings and needs have to be put first, even if only briefly. That understanding of how the world works helps them to be better rounded, less selfish individuals.
I wonder if, when DP took the kids home, he got any snide remarks from his ex ? Quite possibly these may have been along the lines of "not taking his responsibility seriously", "getting out of looking after his children" or "you've caused me loads of inconvenience now". If so, he may now be feeling humiliated and possibly guilty - and though it doesn't excuse his behaviour one little bit, he sees you as the root cause of his disquiet and takes it out on you in a fit of resentment ? .....
.... though of course, any sensible mature adult would have replied curtly to such jibes with "don't be so unreasonable and spiteful, you know what's happened, I'll be in touch as soon as I can to rearrange having the kids" - and then come home to give you a big hug and offer to help you in any way they could.
If he does come back, and if he tries to pick a row, ignore him. There doesn't have to be a row - and frankly, there is nothing to be rowed about period. Please try not to worry about this, or feel any sense of guilt.
I'm pleased Lea's condition isn't as bad as it might have been.
Take care.