Your plants might have died for a few reasons besides the actual heat - after all, it gets pretty hot in Tennessee as well...
The first thing that came to my mind is that jewelweed (in my experience) inhabits edges in forested areas, and a full-sun aspect would likely be too much for them. The same forested conditions in the wild would provide lots of mulch, which would need to be supplied by the grower in the home garden.
A distinct third possibility is that they didn't harden off enough from your seed flats. A fan blowing on the seedlings can help there. Also going into a larger pot before planting out (allows the roots to develop more) will give them a better chance.
Dan, if you're a master gardener and I've been stating the obvious, then please just ignore me.
Steve in Bermuda.
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