Depending on your Region there may not be an Open show running concurrently with the Regionals champs show.
Region 7 (California & Nevada) does not offer that. Because they also hold their own (CDS) champs at that same show. Between the number of people qualified & competing for CDS champs plus the pairs in GAIG Regionals champs, the days are already too full for them to be able to offer Open show classes, too.
But most of them do, so unless you’re in CA, you’re probably able to sign up for a warmup class too.
Another thing to note for a champs show, is that no other rider is allowed to ride your horse during the entire show week/weekend.
Regarding no whips: In my personal experience it’s always best for me to ride the entire test-- or at least major sequences of it-- without a whip, at home. I also give up my whip pretty early in the warmup session before my class. I need to know my horse is in front of my leg without a whip, well before my ride time. I know other riders who’ll carry it throughout the entire warmup and toss it aside just seconds before entering the show arena. I did that once, and the result was not good. It’s something to experiment with well before show day as far as what works for you.