MM,
Quanity: You are fine with amounts you are looking at. Even if pulled pork is the only meat being offered, 1/2 lb. per person is a good size portion. And your shrinkage factor is fine if not a little conservative. I find mine to be more in the 60-65% range. But, better to have too much than not enough.
Pork: In your situation, I would go with the bonless butts. Even though I prefer bone-in butts, it would be safer to go with a non-injected piece of meat. One suggestion though. Talk with the meat managers at your grocery stores and ask what they are bringing in for pork. Sometimes they will get butts in but never offer them whole to the public. Rather they cut them up into steaks or country style ribs. They might have non-injected bone-in butts and they just may not put them out that way.
Pulling and holding: If possible, I would wait until you get to the location and then pull the butts if possible. If this is not possible, make sure you preheat the cambro with HOT water. This will keep the cambro from pulling heat out of the pork while it sits. After pulling, wrap the pan with food wrap and then foil to hold in as much moisture as possible. If you still find it to be dry when you are ready to serve, I use a 50/50 mixture of sauce and apple juice. You probably want to serve yours unsauced but sometimes it just is not possible.