My friend regularly parks in Centara's lot at night when we're out on the town. He just slips the guard some cash and parks unofficiallly. You can park in the Hilton, but it will be expensive.
I was at this same festival last year and it is a pretty nice time on the beach with food, beer, music, etc. Draws a big crowd. Hua Hin is actually doing quite well (relatively speaking) during this crisis time. Weekends are busy with Thais and farang coming down from Bangkok for a break. My friend's bungalow resort in Cha Am was fully booked for last holiday weekend.