@SolsburyHill We were in your position two years ago, but my kid had gotten into one of HYPSM early before finding out he was designated as a semi-finalist for NSLI-Y. We did not accept the offer until the very end precisely because we waited for NSLI-Y decision. He was placed as an Alternate for the Academic Year program, so we waited more until one day before. On that day, some Finalist dropped out, and our kid took the spot. And immediately notified the college that he would take a gap year. My kid has absolutely no regrets having taken the gap year. I actually consider his having participated in the Academic Year Program a more important event than his getting accepted to one of HYPSM schools, because you can always go to college but you only have this chance to participate in the NSLI-Y Academic Year Program. That one year was worth more than 4 years of learning the language at college.
Just delay the acceptance date as long as you can. Most top colleges like it when you tell them you will accept their offer but defer attending for one year.