Sorry, no advice on the high chair......on the hitting of the glass, if he stopped doing it with his head once you used a helmet, it sounds like its for sensory input vs. purely out of protest (I'm not an expert here, just a thought on why he'd stop using his head...because it no longer gave him the input he needed). We had good luck on sensory with a few things but of course it is highly variable per child. We used chew sticks (for oral input), we used an inflatable sensory pad on his seat and we also gave him a couple of hex bugs (these are small mechanical toys that vibrate a lot) for input to his hands. These all seemed to help. By far the biggest success was a weighted blanket when he sleeps. Wouldn't see any reason why it would not help in the car as well though. Just some thoughts.....

I would put him in the middle at the back between the two front seats. Give him things to hold that stimulate him. And keep car journeys short!

https://a.co/d/7FaN3rr
I am quite fond of this seatbelt cover. It keeps your kid from being able to undo the seat belt. Working in a drive thru, I saw more than one mom affix a baby gate to the metal part that is between seat and headrests in front of the backseat creating a barrier. Have you thought about or tried some kind of protective gloves or several pairs of gloves or mittens during necessary car rides if your kid would tolerate that?

I totally understand you I have a car seat still for my 7 year and when he starts to have moments like this he knows how to unbuckle himself and come and wrap his arms around me or try’s to go in the front with me screaming idk what to do either I give him his favorite items I have headphones but nothing seems to work especially when I’m alone with him I feel like breaking down and crying because I have child lock on both sides on the back but I’m scared that if he goes in the front he will open the door hugs to you momma your not alone

We also have an issue with window hitting and it's getting harder and harder we have tried the middle and he's big enough to reach over and still hit the windows so I have no advice but interested in finding out if you had found any helpful solutions to this yet that we might be able to try as well