I think the school is Being VERY Unreasonable (haven't read your other threads).
I would make a very clear list of issues, starting with the more minor things and leading up to (the biggest issues in your mind) or listing them in chronological order.
Revealing your email to your ex was completely unprofessional and could for some people be very serious.
The whole bumping heads thing was an accident, IMHO children should not be excluded for such things, get a copy of their policy on exclusions etc (might be on their website, you could also request one by email before the meeting. Make sure they are keeping to their own policy (they may not be).
The whole thing with 11+ sounds a complete fiasco, get them to tell you what happened, clearly and concisely before you rush in to comment, was there some step missed, in them asking you or informing you or in your doing something.
Always get their side first, perhaps in writing, and then explain how you explained things. Do not reveal everything in one go, because they can then brush it all under the carpet and leave you grubbing around for 'facts'. Get their 'facts' first, make notes as you meet with them. Have your own notes on how you experienced things.
The class laughing at your ds was wrong, did the teacher speak to the children about this at the time. Be sure of your facts. Children can exaggerate and lie (mine do) but they can also tell the truth!
Don't bring in anyone else, rumors you have heard, or things you know about other parents, teachers or situations (I am sure you know this, it will only muddy the waters).
Re " I didn't want to open myself up to further problems with my ex also (they don't keep anything private) but now they have swung round and seem to be trying to set Ds up to look like a hooligan/liar/untrustworthy. "
They absolutely should know what can be kept private and what cannot (e.g. your email address). It may be that they need to tell your ex things about your son, they are obliged to, if he also has parental responsibility.
I do not know your situation with your ex but can you work together at least in terms of how you resound to the school, for the sake of your son? If things happen at school can you tell your ex before the school gets in touch.
If emails are an issue, I would personally investigate a separate email address for school, and another for your ex, and a third for everyone else. This is annoying but very easy to set up.
Re "I do have a tendency to back down to avoid confrontation." try reading up on the internet about assertiveness, this is not aggression, it is assertiveness, putting your point across clearly and fairly, speaking calming. I would pepper my comments with 'How can this be resolved, fairly." Etc. That is not confrontation on your part.
If the other person/people are being confrontational, I would listen carefully to what they say and think about my response. I would not feel rushed into stepping aside. As Saffronesque says I would also try and take a friend or family member with me for moral support if you have one.