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After a lunch break (I didn't eat) and rest, #8 was reasonably easy, but #9 was beyond my physical ability once I'd carried my first load across

I'll bet that low blood sugar might have played a hand in your crashing? I sweat like you do and have found that a combination of heat/humidity effects my appetite. I bet if you would of eaten a few handfuls of Gorp you might not of crashed the way you did.
Still might of been pooped and rightly so considering the conditions, but that sudden drop in energy to me is a sign that your blood sugar might of dropped to low.

My current " job" consists of working 3-12 hour shifts in what they call the Glass Bonding area. We attach the metal components of auto sunroofs to the raw glass with commercial urethane that needs moisture ( humidity) to cure.
Minimum humidity has to be 35% plus and room temp is in the high 80+. The have misters on the roof to keep it that way, and are work area is encased in plastic panels like you see on commercial loading docks.
On a average shift I handle 600+ sun roof sub assemblies that weigh between 30-40 lbs. That and all the racking/gaging and water tests they're respective programs call for.
In 2 years I've gone from last in seniority to 3rd, because people can't take the heat and humidity and the physical strain.
Considering that I'm 30+ years older than the next oldest person, keeping hydrated and eating right is the only way I can do my job.
The " kids" all laugh at " Old School" ( my work nickname), but when they see me work and thrive while most of them wilt, they soon change their tune ;)
Of course I eat healthy, drink the right stuff ( water) and get at least 6-8 hours sleep the days work.
The kids live on junk food, Red Bull/ Monster energy drinks and try to get their rest on the job ::)
You really are what you eat and drink, never so true than when your in the bush and the load is all on you ;D