Where should I dispose watered down, VOC latex paint

Generally the durability is considered to be just as good as the older paints. The VOC-Free paints still have less color-selection (at least last time I looked), but that's been changing as demand increases.

The building I work in (in the U.S.) just completed a LEED-Platinum renovation, and used all VOC-Free or Low-VOC paints. Some of the special purpose paints were not available in VOC-free yet, but low-VOC paints covered most of what they needed (including paint for a metal roof).

I was impressed at how much less they smelled when I stood next to a painter with an open tray of paint for several minutes before realizing he had fresh-paint out (I was disappointed that it smelled like someone had painted a week before until I realized he was doing the touch-up painting right then). In the first year we haven't seen any issues related to durability.

The final color of the paint depends on the ratios of the different pigments in the paint, and those ratios won't change once the paint is mixed. The storage instructions on all the interior paints that I have say nothing about high temperatures, just to protect from freezing which I believe would cause the paint to de-emulsify.

That said, a paint can's lid doesn't form a perfect seal, and higher temperatures are going to cause volatile chemicals (including water) in the paint to evaporate faster than normal. (I've noticed that paint that has been left open for a while and thinned with water behaves differently than a can of fresh paint -- I can't define exactly how, but it just doesn't paint as well). However, given that it can take weeks for a coat of paint to cure fully under normal household conditions, if the can is sealed well, my SWAG is that it would take years before you'd notice a change in the paint.