Finding this thread pretty interesting as im studying Engineering at the moment (only first year though)
This may just be a load of crap but i feel like researching and giving an opinion.
It says on the Rota site that they use A356.2 aluminum alloy I think this is the weakest of the alloys from what i can gather.
The Rota site also says that they use some form of heat treatment, looking at some standard heat treating methods for A356.2 a method is to heat it too 1000degrees F which is 537degrees C then quenched in water 65-100degrees C.
So you said your brakes heated up to 800 degrees with the rim having about 1mm of clearance from that heat. So my guess would be the crystalline structure in the rim would be getting screwed around every time you took the car out for a track day. Im guessing it would be like welding aluminum how it loses its strength when you weld it.
So the rim becomes weaker due to lack of clearance between the caliper resulting in crap loads of heat making the rim become more brittle and loose strength, and then the sudden temperature drop by driving though water forced it to crack, like using liquid nitrogen on something and hitting it with a hammer..
Well thats my guess as i dont see any flexing on the wheel for it to crack that way.
But hey im only a first year student so i might be wayyyyyyyyyyyyyyyy off, tell me if im wrong... if you actually know what your talking about
Just my twoCents