DS has had occasions in school for one reason or another his friends haven't been in due to illness or taking part in events.
This has meant he has been alone. Or they have been alone. On occasion this has been for consecutive weeks. They've all just got on with it.
Tbh, its not healthy to have that level of co-dependence on other children at age 9. She should be able to deal with it occasionally for whatever reason.
Its not ALL the time, even if its a regular thing.
How does she cope if shes at an external group and there's no one she knows? Does she get much time alone?
Is she likely to go to the same school as all her peers or might they end up going to a number of different schools? High School isn't that far off. Its only going to be 18months before you will get High School allocations though. How would she cope if she found she was given different options? Given her disability there is an increased risk here in terms of whether the schools nearby are all accessible which might influence decision making and allocation of schools...
My personal feeling is by Spring of Yr4 you really should be firmly focussed on preparation for High School in emotional terms, because blink and you will be making those decisions - they really are not long off and its much easier to take a longer term approach to this rather than having a bit of a panic in yr6 about it (theres a number of kids in DSs class who really are in this situation because parents have mollycoddled for far too long and then are coming up to the cliff edge of reality).
Honestly, I do think you are making a situation which isn't ideal far worse and adding to the upset by fuelling it with your outrage rather than fostering an idea that this just wasn't her turn, there will be other opportunities and it sucks being on your own occassionally but its not that much of a big deal. By dialing up the drama you create the drama rather than reassuring her.
If shes not coping with this situation thats a whole different line of approach rather than insisting she be allowed to force her way onto the choir. Its a 'would it be possible for her to read in the library instead?' type third scenario solution rather than being pushy and difficult.
You need to teach that life isn't fair. Sometimes unfair decisions are just unfair, sometimes they are done for reasons you don't know but are perfectly reasonable under the circumstances. Sometimes its to give others an opportunity. Sometimes these decision making processes actually work in your favour. Thats it. And you deal with it. And yes she probably will have to deal with it more than most but its not always just about her either.
But yr4 kids should know how to accept losing or not having everything their way. Its something DSs school made a big point at during Yr3 because otherwise it caused all manner of issues - not just with the kids but also the parents.