I just talked to my Liason Officer and he told me that a principal nom does NOT mean an automatic appointment and they can still reject me if there are more competitive candidates on the slate. Is this true?
First off, if you are designated as MOC's "Principal" nominee and, here's a very important caveat, you are fully "Qualified" - academically, medically, and physically - then,
by law, you will receive an appointment.
If you are the principal nominee and you, for whatever reason, fail to be qualified in one of the above three areas, then the appointment goes to someone else.
Again, if you are the principal and someone else on the slate has better ACT scores, better CFA scores, etc. you still get the appointment as long as your ACT scores, CFA scores, etc. have met the minimum qualifying standard.
Principal is a very specific term with regard to nominations. You said "Primary" earlier, which sounds similar, but, IMO, you need to make sure what kind of nominating slate your MOC sent forth to the academy.