This IS going to sound harsh but I am wondering if the friendship split is affecting your DD because you are investing so much time into it and feeding it the problem that, from your own words, isn't actually there!
I have a friend who was very vocal about her son (YR) being kept back in a split Y1/R class when the rest of his YR peers moved to a straight Y1 class. Her son initially voiced his displeasure at this and his mum jumped on the bandwagon and mad it her ultimate goal to ensure that he was paired with his best friend for Y2. Admittedly, she got her way and since then her DS has been with his friends since. But everyone wondered whether this was an actual problem for him or for his mum! Sometimes, it isn't actually our children's issues or problems but we make them into their dilemmas by continually feeding the issues.
You said that your DD gets on well with others in her class. She has play dates with them and she sees them during break and lunch times. So actually, she sees her friends JUST as much as she would do had she been in the same class. It sounds like your DD needs THEM to hold her hand during lesson time and them simply being there gives her confidence.
I told you it would be harsh but I think your DD's problem is not necessarily being split from her friends, but actually her confidence to stand on her own two feet! Especially seeing as you said she burst into tears when she found out that again she wasn't going to be placed in the same class as her choice of play dates.
How is she going to cope at Secondary school? There will be an even lesser chance of her being with her play date friends then than there is now. Even in Primary, there is no guarantee that she would be in the same ability groups as her play date friends or sitting on the same table. So their contact time together in the same class may not even be increased!
I do think that this may not be such a massive problem if maybe you didn't give it as much time and effort into trying to 'sort it out'. It sounds like she is hanging onto false hope that things could change caused by you telling her things that are not determined by you and things that have not even been promised. 'We'll see what we can do' is NOT saying 'We'll change her classes'. Even though you might think, 'nothing has changed' that is precisely IT! Nothing has changed because nothing CAN change. 'We'll see what we can do' is not a promise to change things. I do believe THIS is why your DD is still hoping for something that isn't going to happen.
She, even at Primary age, is going to have to understand that she is not going to be taught in the same class as her friends. She goes to school to learn.
Sorry if this is harsh, but it to me, this is YOU feeding her anxieties and worries. Try not to be so 'invested' in this 'I'm not in the same class as my play date friends'. If she sees that you don't see an issue in this, she may back off. But seeing as this has been going on for so long, you may have a battle on your hands.
Ask her how she is going to cope in Secondary School. if you 'give in' to her teary demands of 'I want to be in the same class as my play date friends', you could end up with the same problem arising again when she's 11.
Good luck whatever you decide to do. If you REALLY, REALLY, that bothered that despite her being taught in a class of children she admits to getting on with and not with her play date friends, then maybe ask the school what their criteria and reasons were for splitting her up from these other girls.