DD1 is still doing well, thank you.
Arrgh...I can't lie to you. Everything you say resonates with me so strongly.
DD1 is going for her first 'play date' away from me this week. She is getting special permission for the taxi driver to drop her off at the house of one of the other girls and they are having tea and play - 3 girls with SN. The mum is quite barmy 
I honestly feel that the difference for DD1, apart from specialist teaching, in being at Special School is that she has friends. Real, honest friendships, where they are friends one minute, enemies the next - just like 'normal' children.
There are no special cases at her school. Wheelchairs, kaye-walkers, standers; PEG feeds, NG feeds or oral food; no speech, minimal speech, complete speech; nappies, pull-ups or pants - the children just don't see it. Their 'normal' is whatever the child is and there are no special dispensations.
In fact, I have to tell DD1 to wind her neck in when she's telling me how rude so and so was for blowing raspberries. She doesn't see that they are non-verbal and that is their biggest communication achievement. She sees it as 'well, I'm not allowed to do that, so it's rude!'. I have to remind her that it's only rude if you can do something different and choose to do it to be cheeky.
DD1 gets celebrated at her school, not because she's got SN, but because of what she can do.
You are her parent. You have the right to push for SS if you feel it's right. Just as you have the right to stay in MS if you feel it's right.
If you don't feel your DD is in a place where she has meaningful friendships, that's something that needs to change, somehow. Whether that be a change in the way the school deals with her, or another school.
That's why I'm thinking that Disability Gym is best - I want DD to start equal, not to be the 'special one.'