First off, I think you have every right to dictate the terms of YOUR child's party. So I would go ahead and plan it on the date that works for YOU and invite who YOU want.
Second, I think your husband needs to stand up to HIS family on behalf of his "new" family. My DH did this. He phrased it as, I am getting involved on behalf of my family.... He said it was not our intent to anger anyone, however he did not expect for ME to have to put on two parties in order to keep THEM happy.
Finally, I think that they have every right to not drive in for the party at your house. That is THEIR choice. If they feel they are missing out, then perhaps THEY can put on a "family party" the next time you plan on coming into town to visit with them? If not, they can choose to send a gift or not. Let THEM handle the details...who gets invited, the games, give aways, food, etc. Since you already had a party, this is THEIR GIG. You just act like the grateful, appreciative daughter-in-law.
I am a big fan of speaking up on behalf of myself and my family. I believe that we have every right to make the rules for our own family. I have struggled on the wording/how to phrase things in a way that I make my intentions clear, however also the let them know that what I am planning is in direct conflict with what they want/expect, I am not doing it to upset anyone and I hope they understand that THIS is what works best for OUR family.
Although I am sure I have upset people, and that they don't always agree with OUR decisions, it has nonetheless happened the way that WE wanted. And if/when people couldn't come, it was clear that it was THEIR choice not to participate, not that they weren't welcome or invited.
Just make sure that you and your husband present a united front so that you aren't blamed as the B*TCH who is "breaking up the family".
I think once people understand that you aren't going to be led around by a leash, then they have a new respect for you as a wife, mother, daughter in law, etc. Again, they may not agree with your decisions, however they RESPECT that you are standing your ground.
Best of luck!