no, unless the secondary specially EXCLUDED all outsiders, the students from nominated primary can only enjoy additional "points" but not an absolute preference during discretionary places stage.
Depends on how close the two schools are. It's because they won't know who didn't apply, but they definitely know who applied. So, if they have access to the complete P6 students' list, then they can know who didn't apply.
Obviously this principal doesn't understand the system very well (and it is perfectly fine because a secondary school doesn't need to understand this).
The 15% is applicable to the seats available for central allocation only. It is NOT related to the seats for the discretionary places stage. And the allocation is done by the computer system, no manual works are required so no ones must know. These seats reserved for the nominated primary schools are only available to these students until they are used up. It is assigned by the random number similar to primary allocation lucky draw, but normally only applicable to band 1 students (since nominated seats are often very limited).