It sounds to me like your HT is very insecure and defensive. Hence when there was an issue that was the school's 'fault', interactions became fraught. IME interactions often are full of tension if one side is defensive. The other side feels it is necessary to say something, and tries to do it in as gentle and positive way as possible because they don't want to set the other side off, but the defensive person still feels attacked and, despite knowing they did wrong, is afraid of admitting it, so lashes out. Even worse if they DID have to admit it - they then have a strong need to distract from their own failings and direct the attention elsewhere. And if the other side had been trying to be gentle and positive in their criticism, they will feel bewildered by this reaction and easy to get cross. They may have wanted only to provide feedback so that the same issue can be avoided in the future, but this cannot happen if the other reacts so defensively. That's frustrating because you haven't achieved any improvement, but have had the experience of an uncomfortably confrontational interaction despite only wanting to help. I can see why you would want to avoid any avoidable interaction in the future.
It's kind of the opposite of a culture that allows mistakes and learns from them.
OP if this is the case, take a step back, try to perceive the HT as a human who not only makes mistakes, but also has a 'fault' in his defensiveness (I am sure you aren't perfect either!). Can you see around that, and perceive his strengths? Which I am sure exist also. That's what I try to do with our hugely defensive HT. E.g. one thing I appreciate her for is that her defensiveness includes the teaching staff; which means that the teachers get to do their jobs free from 'confrontations' with parents who think they know better (which at times includes me; but I'm glad they don't get embroiled in negotiations every time a parent thinks that and accept it means the same rules apply to me).
Then go to that parents evening with the aim to talk to your children's teachers, and if you see the HT, be polite to him. If he should initiate any conversation about your children, don't feel you have to either accept what he says as fact OR need to dispute it then and there; just ask for elaboration and prepare some phrases such as 'I hadn't noticed any of that, I'll have a think about it' or 'Thank you for sharing your observations, I will discuss with DC's teacher to see what they think.'
At the end of the day, what does it matter if the HT thinks your child is shy or whatever? You think about it, consider other people's opinions (the child's teacher), discuss with your DH, and either decide they were right and so move on to what you can do about it, or simply move on. You don't need to convince the HT (or even get him to admit) that he was wrong.