As INTENDED, Horrors could take Daemonology and get the free Primaris.
As WRITTEN, they take Daemonology, get the free Primaris, and get the Change Primaris.
Checked and double checked and I fully agree. Not that I play Daemons/Chaos, but I face them a lot so this is pertinent info for me.
Unless I'm mistaken (and at the risk of starting a whole other discussion) psykers can only cast a number of powers equal to their mastery level.
You are mistaken. While this may have been the case in previous editions I'm still trying to forget, it is not the case in 7th edition.
iven the issues I have getting multiple psykers to cast even a single power each reliably, it doesn't sound all that broken to me
I've faced Daemon lists with 15+ masterly level points. They do what they want in the psychic phase