Horrid Henry was debating who to invite to his birthday. None of the children were nice to him, but without invitations there would be no presents. 'Oh bother, let them all come' he decided.
You aren't Horrid Henry . But the advantage of inviting everyone, is that they each may find they need to offer something in order to justify their attendance. And having health, education and social care in the same room does at least stop each one saying it's the other service's job to sort things out.
As justa says, having a wish list of what you need from the meeting is helpful. The key to success for any of these is really to have a clear agenda, a minute taker, an advocate or friend to back you up and someone helpful to chair the meeting. At best you could walk out with a clear recognition of who thinks what, and what type of help will be forthcoming in the next 6-12 months. And just as usefully, you'll know what you'll need to organise and resource yourself.
If your worst fears were realised and all concerned were to gang up on you and say you're a label chasing loon, whose obsession is harming your poor dc.... even that would help a lot. It would give you the prefect excuse to insist they all work together to get a very speedy, thorough, specialist multidisciplinary assessment whose findings you could all rely on (and you obviously know that a full evaluation should demonstrate the issues you've seen).