Weddingbee boards

My fiance and I both have big families, but while I am very close to most of mine and they all always travel to family events, he isn't particularly close to many of his relatives and a lot of them don't travel even to weddings. As soon as we got engaged (over a year ago), I started asking his mom about how many people she expected to come out to an East Coast wedding in Fall 2012 (they're mostly from the Midwest) and couldn't get a response out of her. Last Easter I got her to start asking her relatives at their family party, but everyone apparently just said that it was too far out to plan and they couldn't begin to know whether they might be able to fly out for a wedding a year and a half away. Eventually, once we had started looking for venues and really needed to know how many people to expect from the Groom's side, his mom finally told me that she would anticipate at most 40. With that number in mind, we chose a reception venue that holds a maximum of 200 people. As of now, my parents are paying for the entire wedding. The Groom's family have said they might be able to contribute some money, but haven't committed to an amount or even spoken to my parents about it at all. 

As we were getting ready to send out save-the-dates, my parents came to the decision that they were going to allocate 45 people on the guest list for the Groom's family (extended family: his parents and siblings are separate). When we explained this to his mom, she was furious and denies ever having given me such a low estimate. She now would expect at least 80 people to be able to come! First of all, we have a hard limit at the venue, so there's minimal flexibility on this just in terms of numbers. If she had told me that number a year ago, we could have picked a larger venue, but even then my parents wouldn't have felt comfortable paying $250 per person for more than the 45 they already allotted and would have needed his parents to contribute, which they don't seem willing/able to do. 

His mom seems to have resigned herself to the 45 limit, but now there's a conflict over save-the-date and invitiation etiquette. My parents and I decided that we don't want to rely on guest list attrition to meet the 200 limit, so we are going to send out invitations early and to no more than 200 people. If people RSVP no, we will then send out a second round of invites (still a couple months out) to people who didn't quite make the A list. Since you need to send an invitation to everyone who gets a save-the-date, we are also only going to send save-the-dates to those 200 people. The Groom's mom is now insisting that we need to send save-the-dates to her whole family, and that she wants invitations to go to the people who tell her they won't be coming. The issue here is what if they do decide to come after all? We can't take the risk of inviting someone thinking they're not going to come, only to have them change their mind later and screw up our numbers. Her family always sends invitations to everyone for weddings, but they also always have weddings in huge venues with buffet dinners. We will have room to seat exactly 200 people, and there will be a sit-down dinner. We don't have the freedom to have an uncertain number of guests.

My fiance and I are getting stuck in the middle of all this, since his mom refuses to return my mom's calls and speak to her directly about her concerns. Any input or advice would be greatly appreciated! Thanks!