Sat. July 20th Heart concert is going to have record high/dangerous heat levels, is there a chance it might be posponed?

I'm really not sure. But I doubt it. I would call them. Here's a link to the Xfinity site. http://www.hartfordtheatre.com/comcast-theatre/

I have been there in almost 100° weather they did not cancel the show. They did have cooling stations around.

Never been on the lawn. Does anyone "think" my chair will be turned away if it's not a low one? Have a medical issue. Are they super strict about checking the chairs?

Absolutely not the show will go on rain or shine have fun , where light bright clothes.