I totally disagree that "we'll make it work" is a verbal RSVP in the positive. Talk about stretching the English language until it no longer has no meaning, that's it right there. "Make it work" does not under any guise of normal conversation translate to "we well absolutely attend both your events" especially when its paired with "the date doesn't really work for us" as it was. "Make it work" could easily mean we'll attend one event but not the other, or we'll take you out to dinner some other time, or we'll send a written toast to be read to you. To me "make it work" = "we'll work something out" and the solution is not going to be the original request (which in this case was attend both events), its the offering of a compromise, not an affirmative RSVP.
Especially since we don't know the full conversation. Maybe BIL and FSIL live out of town and are coming back 'home' for this party, the original email/conversations could have been something like "we'll be in town for an engagement party the weekend of Saturday May 11 and then our wedding will be the following year on Sunday May 11..." and the LW and her DH had plans to go away for the weekend, so the "make it work" comment easily could have meant "we'll put off leaving until Saturday morning and travel on our anniversary so we can see you Friday night when you get to town".
Without knowing exactly what the DH was responding to, we cannot make absolute conclusions on what his response was exactly referencing.