This is similar to the post about not knowing the husband below, but that post did not answer what I was wondering: if I know the wife and only know the husband's first name, and know the wife has a different last name than the husband's and do not know the husband's last name, how do I address the envelope? I understand I invite them both, but how would that look? And finding out the husband's name wont work for me- I am a teacher, and wont see the wife again (fellow teacher) until September.

I hadn't foreseen this problem until now...
Also, I am inviting two other women whom I do not know their husband's first names (also teachers I can't contact about names)- would that be just "Mr & Mrs LastName" (due to lack of the man's first name)?
And last, this is just to confirm what I already have seen, your return address goes on the back of the envelope, right? (Not the upper left corner like on normal letters) or does that not matter?
Thank you!!