I am just a mortal mid-pack runner doing my training and racing in the sub-tropical conditions of Hong Kong and would say we know a thing or two about the effects and interrelations of heat and dehydration. The sensations you describe are still more consistent with some level of dehydration and a subsequent impaired heat tolerance, resulting in a massive bonk that can simply pull the floor out from under you in a matter of minutes. Frequent urination and urine coloration are no reliable indicators of someone's hydration status, little to no urination is as bad as too frequent runs into the bushes. In fact, low sodium levels due to the effects of dehydration reduce the body’s water retention abilities and hence result in more toilet breaks, amplifying the issue. Hyponatremia in a hot race is really unlikely imho as you can hardly hydrate enough to compensate for the water loss which can easily be as high as 1.5–2.0 liters per hour depending on individual sweat rates. And if you cannot really overdrink in these conditions chances are that depleted sodium levels are rather the result of dehydration. A relatively low heart rate in these scenarios is consistent with at least my own experienced heat bonks. I found this article pretty helpful btw: http://www.humankinetics.com/excerpts/excerpts/dehydration-and-its-effects-on-performance