If I book Premium View for Fantasmic & the park is at full capacity will they let me in??
No, instead they will refund your money. When they have to shut the gates, it is due to legal reason (Fire Marshall), and they can't let anyone else in.
By the way, it is Thursday at 8:10 AM and there is currently major traffic jams on the I-5 near Disneyland due to the Spring Break crowds... So get there early and be prepared to stay inside the park if you don't want to get locked out.