I see that your ex is a class act.
(Like, it only sucks for
him to get every other year? And that a 7 year old should "suck it up, buttercup".)
I believe your letter would have been more effective if it had been sorter and contained more declarative sentences, fewer squishy ones.
Example:
"Attached are messages received [date] from [whoever - school counsellor/teacher]."
Nothing about "I feel" "I think" "willing" - there is no "try", there is only "do" and "not do".
If you end up in court, ask for a modification that parents use a form of communication that is admissible in court, and then stick with it.
Just start communicating only with dad. You don't make a big announcement or give ultimatums - just do it. Stepmom texts you, and it's not an emergency? Send text to dad, replying, "RE text about [whatever]: ". If he continues to say he's delegating to stepmom, and you need to co-parent with her, respond, "Actually, I don't. She's not the parent. I need to co-parent with an actual parent."
About counselling: take the school's recommendation, and get the kid into counselling
during your parenting time. Make sure that they know that it's during your parenting time, and that Dad and his wife don't have standing to cancel appts during your parenting time.
About a group meeting at the school among parents and stepparents: respond to the request, with a cc to Dad, stepparents, stating that you would be open to such a meeting. Let Dad and stepmom hoist their petards or gracefully do the right thing - it's their choice.
Perhaps your school is very attentive, or maybe things are even more out of hand than you know. My kid's schools have never noticed anything, or offered anything -
and I've actually asked for help - they seem content that the kid is performing well above grade level, and minds her own business. If the notes to you from the school were unsolicited, as a defense measure I would follow through.
Let the school counsellor know what steps you are taking to follow through with the recommendations.