Book the tickets individually
i can’t answer your question but I can give you this advice, even though it sounds like a PITA. Book all tickets individually. Not only will it be less of a hassle to cancel somebody’s ticket but it’s also cheaper. If you try to buy qty 15 from one destination to another it will most likely be more expensive per seat than if you buy them individually. This happened to me last year. As the inventory of seats goes down, the price increase per seat (makes sense) and if you cross that magical threshold all seats are priced at the higher level.
If you don’t believe me, open two web browser and try buying 1 seat versus 15 and look at the price per seat.