Ahoy!
Awesome batrep as per usual. Nice to see your Cult list start up as I've been looking for inspiration to get my wyches off the shelf. Splintermind is a great way to start off a match, hey?
Which section of the Chaos FAQ are you referring to?
When it comes to 7e and CSM psykers my experience and research has revealed this;
-A Sorcerer with Mark of X "
MUST roll at least one, and may roll up to half, of their powers on the table that corresponds to their patron deity." (p.70 Chaos Space Marines codex)
-Psychic Focus only applies if ALL a psyker's powers are from the same discipline. ONE exception; the
Force psychic power is known by all psykers with force weapons, and doesn't count against Psychic Focus restrictions. (p.22 The Rules, beginning of the Psychic Phase section)
-There is a specific paragraph in the Psychic Focus section (p.22 The Rules again) called
Chaos Psychic Focus that states "
If a Psyker has a Mark of Chaos... that model automatically knows the primaris power of the discipline that corresponds to their patron deity, in addition to any other powers it knows."
So in the above example of a Sorcerer with Mark of Tzeentch:
-HAS to roll at least 1 (max 2) powers from Discipline of Tzeentch
-Automatically gains primaris power of Discipline of Tzeentch due to Chaos Psychic Focus rule
-Then gets to roll remaining 1-2 powers (assuming level 3 psyker) from Biomancy.
-Since ALL his powers won't be exclusively from Biomancy, he doesn't get the Biomancy primaris due to Psychic Focus rule.