I live in zone 5 and the place where I found a thicket of wild stinging nettles, was near the river in the floodplain in the shade under lots of mature trees. I'm not sure if they're the same kind of nettles you have purchased, or if they need similar environment.
You can see with only one eye open, but you'll probably run into things and stub your toe. The big picture matters.
I thinkkk they'd be okay with the heat as long as their roots have a steady supply of moisture. They naturalize in areas like creek sides, ditches, banks etc. Anywhere where the water is and the soil is fairly rich.
Check here for all the info you could ever want about nettle reproduction.
Yes to shade and they need lots of water. I don't know what "dioica" refers to but I am pretty certain they are not a dioecious species, all of the plants I have grown or foraged appear identical I have not noticed any thing that would distinguish sexes
I'm in England, so take this for what it is (experience from the other side of the world!). Here, even when we've had what was for us hot and dry summers (a couple of months of virtually no rain) the nettles were one of the plants that didn't seem to wilt very much, at least in a shady woodland environment. In our garden they are also one of the few things we don't water and they are never fussed by that.
Incidentally, as the great article posted above mentions, you can also eat the seed - this could be something you could do as well as harvesting the stems for fibre? A herbalist once told me that the seeds act as a stimulant and a friend of theirs ate some and was up all night, very much alert!
Most people here are at best indifferent to nettles because they're everywhere and, of course, they sting. People find it funny when I say that they are under-valued and that people in other places actually purposefully plant them. Silly because our ancestors used them a lot! Archaeologists think that, because of how incredibly thin some of the nettle fibres they have found are, that much of that work was probably done by children.
We don't have time for this. We've gotta save the moon! Or check this out: