I have the same problem with both my children's birthdays since it conflicts with a couple of her friends and Labor Day. Our solution has always been to coordinate and move it away from the other conflicts. I always tell my kids that we have their Birthday party and their family birthday. The family birthday only includes our family and we let them choose a restaurant and they get all gifts from us on that day we try to make it on their actual BD if possible.
I agree with the other responses. It makes me CRAZY that no one RSVPs. I always get the "I was meaning to but...." response. I would call the other 6 children so that you know for sure what you are facing before you make a decision. If the party doesn't work, then ask your child what he wants to do. You might be surprised with his response.
Even though you asked those parents in January, they may not have realized it was spring break. That was probably too far out and with the busy schedules, they probably didn't check. I have found it is impossible to plan anything around the major school breaks. Good luck :-)