As an all-star dad and high school coach I could write you a book on this one.
Preparation: very dependent on the HS program and how they place athletes. Do they take everyone, are freshman only allowed on JV, do they teach the tryout material or have clinics, etc. So that's honestly a very fluid question to answer.
To the question of commitment and flexibility....my advice is to talk to the heads of both programs before committing to either. At the school level I'm very flexible for all of our kids other activities with some exceptions. Friday nights (or Mondays for JV kids) are never going to be excused for anything other than illness/family emergency/etc. not for all star, other sports, school plays, whatevers. Additionally, Thursday practices are our "tune up" days. All stunts, tosses, pyramids, tumbling that will be used in that week's game will be practiced that day. If you can't be at that practice you can't be in those elements. Just like how it is in all-star where you can't miss a practice, especially on a comp week, without repercussions like being pulled from the mat.
So make sure there is a clear understanding of what the heads of both programs will allow or not allow when it comes to missing a practice or event. Most high school coaches know the local all star owners well and they're all comfortable sharing schedules and finding ways to make things work. There are exceptions to every rule though. I know high schools who do not allow their kids to cheer all star during the high school season, through nationals.
I'll pause there since that's what you asked....but feel free to throw specific questions and I'm happy to keep answering (from my perspective)