I'm not an expert. There is SmokinOkie's Pork But 101 article you should read, and gobs of threads on this forum.
I can't answer all your questions, but here are some of my thoughts:
1) Use plain white buns. I like the giant sized hamburger buns, but regular size will work better for kids and stretch your meat further for adults who want two sandwiches.
2) For 12 to 15 adults only (you don't mention how many kids) I would do no less than 16 pounds of pork butt. About 40 to 50 percent of the butt will render out as fat. If there are kids and especially teenagers, you'll need more than you might think. I count two young kids as one adult. Teenagers count as one adult unless they are male, in which case they count as two adults. Yes, really, they eat that much.
3) Figure on 1 to 1.5 hours per pound of pork butt, based on the heaviest one you are cooking, not the total weight. So if you have one that is 10 pounds and one that is 8, figure 10 to 15 hours.
4) Most stores will label a Boston Butt as Pork Shoulder. For a big group, stick with boneless pork shoulder. The picnic section has the bone in it. They work fine, but you'll need more of them to compensate for the weight of the bone, and consequently you'll need more grill space.
5) Avoid meat that comes pre-brined or injected with a solution. You want minimally processed pork shoulder, no additives.
That's about all I can think of. I tend to way overcook for parties and other moderate sized gatherings because I'd rather freeze or give away leftovers than run out of food. If I was planning for 15 adults, 8 young kids, 3 teenage girls, and 4 teenage males, I count that as 30 adults. I'd cook no less than 32 pounds of boneless pork butt. I'd be more likely to go for 40 pounds just to be sure. Say, five of the large half-butts you'll find at Costco.
And make lots of sides.