actually I was expecting the typical, no one rsvp-ed question that is asked all llllll the time, because people are rude and don't rsvp any more.
have to say, if I were the other mom and you called me, I would not be willing to combine bdays, and at this point after I sent the save the date, I would not be willing to change it -if it were just a few friends I might have changed it but not for the whole class. which I understand is why there is conflict.
sooooo that brings me to two thoughts, #1 if she isn't organized to send actual invites then as a mom of a classmate I wouldn't "plan" on attending her party until I got the actual specific details, time, place etc. I couldn't tell from your post if her email actually was a full on invite that she scrambled to email out in response to yours but was trying to play nice and call it a save the date. so that would be a vote in favor of your party because it's clear and set.
BUTTTTT #2 as someone else mentioned Sunday morning is church time, and even if it wasn't it would be family time, so I would probably decline a class party early on a sunday unless you were a super great friend and we had the option of going to a different service or something like that, so that is a vote against your party.
What I would do would be to FILE this away so that NEXT year you either have a talk w this mom ahead of time, or pick another weekend to celebrate. this year, I would probably invited a few friends from outside of school so kiddo was sure to have some pals there,
AS long as, say 3 kids show up, I doubt your child will mind a whole lot unless you make a big deal about it, and I do understand how awful that would make me feel as a mom, but I really don't think this woman had a personal vendetta against you and your kid, if you can think of it that way, it was just something that happened and maybe next year it will be different.
and finally, if I felt like it, if my kids cared enough, if I had atleast heard nice things about these kids from my own, and if the time didn't conflict with our standing obligations I probably would attend 2 parties, can't really remember those starts all lining up though.