I understand here in your posts than once I have start to use the reserve fuel of 30 minutes at destination, I can not fly to alternate anymore?
what happen if I accept a holding clearance at destination and I hold for 15 minutes(by example), and suddenly I am not authorized to land cuz the plane in front of me crashed his gears on the runway?
my only way is to go to alternate, with the rest of 15 minutes holding ...in this case I can declare a "PAN PAN , we are short of fuel"!