I had a very detailed topic about our possible starting issues about 3 years ago :
http://www.m3post.com/forums/showthread.php?t=1142234
If that was the starter issue what I don't understand is how come BMW Service can make a comment like it can be either CAS or Starter? It is very easy to find this out using the damn ISTA!
In order to find the problem, when this NO START occurring, look at the terminal 50E and see what is the voltage. If Terminal 50E does get the juice (12ish Volt anything lower battery having some juice prob) while you attempt to start and starter just clicks it is clear that starter is dead. . And the Volt has to go back to 0 after the car started. If the value is still higher solenoid is not doing the job anymore!
https://www.youtube.com/watch?time_c...&v=9NBxDfOFfwQ
6:15 watch the terminal 50E while car starting! Because of your only starter having random problem, you can see that values while you are attempting to start.
As far as the CAS goes, if u re having problem with the Cas, ISTA will tell you cas can not read the terminals. I had reach that terminals using the CAS module!