Shiner is correct as usual, though I would add that if you validate classes, you will be validating mostly core classes so your timetable is accelerated for getting into non-core classes. You still won't be hitting electives your first year, but you may be hitting some classes for your major by Spring Semester of your 4th dig year. I won't rant more about scheduling since I have done so before, but realize you have to champion your own schedule to get the one you want.
As far as your plan B, we went as far as putting a deposit on his Civilian School of choice. It was $500. If he got turned back, or otherwise left BCT, he could still show up to Plan B since they didn't start classes til late August and BCT is over by early August so you will know if you need it or not before civilian school starts. I don't like blowing $500, but I thought of it as insurance and given COVID last year, felt like protecting that option more than I normally would probably. I rationalized the expense by telling myself that if he does not need Plan B, he has a full ride at USAFA, so I will have only paid $500 for his college which is not a bad deal.