I took your wonderful advice when speaking (aka having my fiance speak) to my FMIL about inviting non-attending guests. The venue capacity is 180, and we are not changing the venue. She exceeded her 33% and is inviting 80 people, 40 of whom are expected not to come because of distance, age and/or estrangement.
However, FMIL is very adamant that we invite the 40 people who will not be attending of the 80 people on their list because it would be very hurtful to them should they find out, and that we should invite 220 people to make up for the 40 not coming. Plus, they're keen on the gifts that these relatives will likely send, and these relatives have been invited to (and not attended) other cousins' weddings.
I'm fine with inviting some of these relatives that they talk to regularly and the few that FI and I have met, but I'd really hate to cut close friends in favor of invitees who will most likely not come and whom FI and I have never met - and then have only 50% of invitees show up to the wedding. That would be a bummer.
Strategies used so far:
1. Sending articles on why we cannot invite more that venue capacity - she said these guests just won't come anyway
2. Having FI talk to her - he just gave in to her (Momma's boy

), and he doesn't quite understand the capacity thing anyway
3. Giving her a set number of guests to invite - she said 40 people won't come anyway, so she was technically under her 33%
4. Asking if she has friends that she'd like to invite instead that might actually attend the wedding, since we're hoping for a good turnout - she said that she'd rather invite the relatives
She is SO nice, which makes it difficult to be really strict with her, but I think it's her niceness that's keeping her from scratching people off the list. I'm sending out save the dates soon and would like to have this settled quite soon.
I'm thinking of trying #4 again and insisting a little harder this time... what do y'all think? Thank you so much in advance!