Back in my teens I saw a few Air Cadets face plant after standing on parade for an hour, in high temps, in full dress. We were told that locking your knees might be less tiring, when standing for long periods, but it contributes to head meeting parade ground.
I've never experienced anything as bad as you, thankfully. The worst I ever experienced was when I was working in the racing pits, for a ladyfriend. Her sponsors didn't show up for a weekend when it hit about 34C and I ended up doing all the go-fer work. Forgot to drink anything, for extended periods. I don't even remember the 2.5 hour drive home (I'm an idiot) and flopped into bed when I arrived, then spent the next 2 days off work and taking in almost exclusively fluids.