Weddingbee boards

I think you should use what they would feel comfortable with!  That was my approach to invites--women who I knew wouldn't want to be Mrs and Mrs Thomas Smith were Mrs. Susan and Mrs. Thomas Smith, etc.

Most divorced women I know would rather be Mrs Susan Smith (or even Ms. Susan Maidenname, if she's going by that now) than Mrs. Edward Smith.  Some widows (esp. very senior ones) would go by Mrs. Thomas Miller, but probably wouldn't mind if you called them "Mrs. Better Miller".

I would never call my single/unmarried friends Miss, but because they would probably kill me--and I wouldn't have used Miss for myself either.

A lot of that is regional/cultural/family specific, so maybe the best bet is to ask your mother or FMIL what those specific women are going by.

Good luck!

Doctor Girl:
Dr. and Mr. Smith (or Dr. Susan and Mr. David Smith or Dr. Susan Smith and Mr. David Jones)

We went through all sorts of title-ettiquette issues for ours (including people with WAAAAY too many titles!), and no one complained (except a recently graduated MD who joked that our reply cards with M__________ weren't very helpful for Drs, Revs, etc.--so she crossed out M and wrote DR in BIG letters).  I think, as long as you are respectful and do your best, people will appreciate the invite. :-)