Again I have to say that the authorisation process isn't infallible - it is one of the main areas that is discussed on military Flight Safety courses - and once the aircraft is airborne it is up to the person with their hands on the controls how the flight is actually conducted.
The authoriser is a check and balance in the aviation process and can assess if the sortie is possible under the conditions he is briefed on. If the weather changes en route it doesn't mean the pilot can charge on regardless just because he has been authorised.
I concur with your thinking regarding what questions you would ask as the authoriser - that is exactly what someone could have done with this Squirrel crash - asked the 'what ifs'.
The low level routes into and out of Valley are strictly adhered to and were created to deconflict Hawks from Mil rotary. I have seen very few CANPs in Snowdonia and only ever for USL work repairing the paths. Even civilian helicopter clubs doing mountain flying training tend not to CANP their activities.
The students giving met briefing is a standard part of the military training system.