I guess I will just share my story again as one of the reasons why you shouldn't send save the dates to everyone on your list...
The short story is, FIs mother passed away during our engagement. However she had already given me her "list". She said she didn't expect everyone to come, everyone is out of state, and she seemed amenable to not inviting everyone on it. I never asked her about specific people because I did not get the opportunity.
We sent the save the dates several months after her passing. I originally planned on just sending to my family, fi and my friends, and FIs close family. Several people on the list from his mom were people he didn't know. But he has never been as close with his extended family.
My mom thought it was weird not to send save the dates to everyone. So we did (first mistake). A lot of the list didn't have everyone's names (kids or spouses) so we sent to "mr and mrs last name" and his family said that they'd give us the rest of the names at a later date (second mistake).
Now invites are about to be addressed and stuffed to go out in September and we still don't have first names for FIs moms list. Also I found out it includes far extended family (third cousins?!) and people that no one else has met (were connected through his mom only I guess?). Finally, she had her counts sort of off (mainly from saying Billy and Sue and their three kids... But the kids are actually adults in their 20s or even 30s). So.
We don't have a space issue- because we are perfectly comfortable telling his family that we cannot accommodate adult kids living outside the home and their families. And we don't really have a cost issue, though I am not super excited about so many people we don't even know attending.
I wish we hadn't sent save the dates to these people- but I thought they were first cousins (we invited all my first cousins, as I am close to them), and it was still such a sore subject to even suggest cutting his mothers list.
I am mostly just sick at the thought that so many of his family members will think we are being gift grabby by inviting people whose first names we don't even know! I have tried to explain to his family that we need full names- Mrs. Whatever and husband isn't enough- (like we dont have husbands name!) but they don't get it. I'm having a hard time being polite with his family because I think they're being so rude and gift grabby. Most of the invitees are out of state and admittedly won't come. But I would feel so awkward if I got an invite to the wedding of a third cousin id never met!
And just to confirm, FI is the one actually dealing with all this for the most part. But he doesn't see why it's wrong to invite people assuming they won't come, or to invite people without their first names. I'm the one worrying. ...as usual

For example, we sent a save the date to a female cousin and later I found out (...when she visited our web page and signed the guest book...) that she's married! So I feel awful for sending her a std just to her at her maiden name.

she's older and my googling tells me she's probably been married for some time...
I am mostly just venting here because I don't want to overload my FI (and I am so pissed with his family for taking well over a year to get me this information) or my friends with wedding complaints. If everyone shows up I know it will be fine.
But what do I do if we don't get the names? My moms friend is doing calligraphy and needs to start by Monday to give my mom time to stuff things and mail in early September. (Long story but we are long distance planning and my mom graciously volunteered to do the manual labor of putting together the invites!) I know if we don't get their names by Monday my mom can just hand write it in- but I still planned on sending in early September (since 90% of guests are OOT and everyone keeps asking about it). Just not send their invites until we get it?! Then it will look even not like B listing! I just feel like I've maybe offended even more people by excluding spouses or using maiden names, since we didn't have his mom around to help. Oh man.
My mom thinks if we don't get names we can cut them. I know this isn't right if they for a save the date. But what about unnamed kids? Can we just cut them if FIs family doesn't give us their names ?
If you made it this far, thanks for reading... I feel a lot better now.
And be careful when you send your save the dates!!!!!!!! Seriously.