Your temps could be an issue, but humidity also sounds like more of an issue, with regards to the fog, with pachyies. Do you also get a lot of rain?
In the greenhouse I had them originally set up with overhead misters, and I lost the most due to humidity being too high in the house. Never really lost any due to low temps before.
When I switched to drip watering or hand watering they became much healthier and happier. I know this might not be a possibility for you, but just an observation about leaves/caudices being wet and too much humidity for them.
I think altering your mix could be a solution, to some species that like very fast draining soils. Do you prewash your pumice? And what is the size of the pieces of pumice? With my stone soil with pachyies and other caudiciforms I prewash the stone cause it removes a lot of fine particles that get stuck in the soil, when not washed can prevent as fast drainage.
Also can you explain what you mean by your observations from growing from seed with brevicaule and lamerei? Are you comparing these two to each other with drought tolerance? Or too individuals not from seed. Cause those two species IME have totally different water needs and drought tolerances, my brevicaule only gets watered by hand, and nowhere near as often as lamerei which gets watered twice a week. And I've only ever grown from seed so I'd love to hear what you observed.
