S.T.
Most men are not much help with this kind of thing - don't frustrate yoruself with expecting that he'll be much elp with the details. It's not that he's bad (he may be but I don't think this is the indicator) they are jsut clueless with these kinds of details.
One balster is no where near enough. The minumum would be two - but 3 would be better - and how about water guns as party favors for the boys to use while they're waiting to use the marshmallow blaster? AS for points vs jsut shotting hte bad guys - I'd jsut go with shooting them - the points thing is tough to track, there will be errors, etc. All they want to do is shoot marshmallows anyway.
When my son was younger we had a few "supersoaker" birthday parties where the invitations instructed kids to bring their supersoakers if they had one - party favors were water guns from the dollar store and we'd spend the night before filling water balloons and load up a few buckets, wheel barrows, bins with water balloons. The boys had a riot - no rules - just running around the back yard having a ball. Now my son's birthday is in July so it was always warm - I don't know if that's an option for you.
Best of luck with this - remember, we moms tend to make these parties into bigger than life things, we fret about the details when all the kids really want to do is have some fun, eat cake and ice cream, drink some soda, get sugared-up and spend time with their friends.