There is some discussion about temperature tolerance in Park Nobel's book on the environmental biology of agaves and cacti.
The nighttime respiration of CAM plants relies on their stomata opening at night, which is when they fix their CO2, and this process is temperature dependent. The optimum temperature for these plants to breathe is about 20°C (68°F), assuming they are spending their days in greater warmth. Above about 25°C this process becomes dramatically less efficient for agaves.
So that is one effect of nighttime temperatures on plant growth. It does not necessarily explain the crisis "meltdown" response but rather a breakdown in normal metabolic activity, or a loss of efficiency in the tasks of everyday growing and energy capture. Which may sensitize plants to meltdown if they are subject to additional environmental stress related to water.
Temperature has all kinds of crazy effects on plant cells other than stomata, but one thing that stands out is that sensitivity to high temperature can be reduced by chronic exposure. So just like with cold, the harshest temperatures are the ones that are the furthest away (quantitatively) from the running average a plant may be experiencing. A baseline of moderate heat results in plants that are more tolerant of extreme heat, if that makes sense.
Another interesting thing he brings up is how overwatering (or presumably more frequent water delivered by mother nature) can override the usual switch to nighttime (CAM) respiration. In other words, a plant which would otherwise breathe at night (the example given is Agave deserti) may switch to normal (C3) metabolism and daytime respiration if it is given lots of water, regardless of other conditions. This may not be healthy for the plant, but it is a predictable and measurable response to excess water in the soil.
Higher temperatures will promote nighttime respiration, but even plants that mostly follow the CAM pattern may switch over to a daytime mode given very cool temperatures (like what we experience here). The simplest case of plants being C3 or CAM (but not both) is typically not true for many of the succulents that have been studied, which can be coaxed one way or the other, depending on conditions. I suspect that many or most of the plants which are fully CAM at Thijs' place right now would be partly or mostly C3 on our patio here given the mild summer temps.
Finally, just a word to the wise, the point of greatest vulnerability with respect to heat tolerance is usually the roots, when a plant is growing in a pot and not in the ground. Especially if the sun hits that pot and warms it above the ambient temperature. To the extent you can shield your pots (the actual containers) from direct sun for some or all of the day, you may be able to coax greater heat tolerance out of your succulents. Or to put it another way, even in our mild climate, I can put plants in a position where they get uncomfortably warm and start complaining loudly, because I left their pot in the sun and it turned into a casserole.
