Helenium in my experience are not particularly drought tolerant either.

"Misting" is generally not going to provide a lot of water to the plant, in particular the roots. I would suggest letting the hose run at a very low rate at the base of the plant each morning for 30min during periods of sunny, dry weather. Conversely, run a short soaker hose.

Are you saying that you don't think that helenium would not do any better with the heat than the heliopsis? If so, I don't want to spend the money on the helenium.

By the way, I do water the heliopsis well. Water the wall behind, also, to help with the radiant heat.

I wonder if it would help for you to plant something to shade the wall? Perhaps a vine, or a tree or large shrub that would protect it from the heat of the day.

You might want to try lavender. I've got a similar warm spot by my garage where the heat builds up and the lavender loves it. I don't water them at all.

You might be better with a plant that likes mediterranean or southwest type growing conditions.
What about echinacea, agastache, yucca, gaura, lavendar or ornamental grass.
Is the soil there naturally quite dry as well?
Flora