Great question! You can school cross country on its own, but there are no cross country shows. You have to do all three phases. With that being said there is a new movement called Cross Country Equestrian Association that is trying to make cross country it's own sport. It's new and I'm unfamiliar with it but seems to be gaining traction and you should check it out on their website crosscountryequestrianassociation.com/ as it might be what you are looking for ☺️
I am a little unsure of your question at first but the maximum length for a whip in Eventing is 30" (75cm). Whips can be shorter than this. I hope this helps!
If you are able to control a horse out in the open and are jumping already then I don't see why someone wouldn't be. We all progress at our own pace and riding out in the open over solid jumps should only be done when you have control over those same jumps in the arena and know how to control a horse in the open.
