The Dr. compels me to mention that the "heat index" is a calculation that attempts to measure the impact of the temperature and the humidity on a human being. Thus the "heat index" indicates how hot it seems to people. Please be assured that your sedums, sempervivum and jovibarba don't react to the heat index the same way we do.
I've long felt that the hen & chicks, at least, are dew harvesters. When the humidity is high, and you get a nighttime temperature drop, there is the potential for the plants to "self-water" in small amounts by collecting the dew that condenses on the leaves and directing it down to the crown and roots.
The direct heat from the sun, i.e., solar heating, is probably more significant to your sedum's plight than the actual heat index We should all remember that plants in direct contact with the ground will be more resilient to problems with heat (or cold). The ground functions as a buffer, remaining cooler during hot times and releasing heat during cool times. Damp concrete also works in a similar way, by evaporating moisture contained in the concrete, it will conduct heat away from the plants (cooling them) as it evaporates. Thus, you can keep your plants in the fountain cooler by keeping the concrete damp. For example, keeping the fountain itself in a tray that could be filled with water or similar strategies would help. Filling one of the tiers with water, instead of plants, would be another strategy.
