How to address wedding invitations - family with children

To answer your first question, I don't think married couples who don't share the same last name should be offended if you are using their own last names on the invitation.

To answer your second question, I would say it would be between A & D

1.) You should address the envelope just like a couple who lives together and isn’t married. It’s impossible for us to know what will offend them, so I would just follow etiquette.
Mr. John Doe and Ms. Jane Smith
Child One Child two.
2.) Adult children should receive their own invitations. One for parents, one for single adult child, one for adult child and boyfriend.

Why would you go through the effort to use unique names for unmarried couples living together but not give the same consideration to married couples with different last names?

I'm not planning on changing my name after marriage and while I generally wouldn't get offended if I was ever referred to casually as Mrs. [FH's Last Name], I am making a conscious decision to keep my maiden name. I feel like if you would take the time to get to address unmarried couples using the correct last names why not show the same respect for someone who is married and chose not to change their last name?

I feel it's one thing to make a name assumption (like a school teacher calling the parents of a kid Mr. or Mrs. [Kid's Last Name]) but it's another to just refer to a woman by her husband's last name if you know she has a different one.

Using the name a person has chosen is a sign of respect. We are using the legal last names of every guest on our list, regardless of their marriage status. It didn't take that long to check in with all of our recently married friends to determine which last name(s) each person is now using. I feel that if these people are important enough to us to invite them to our wedding, I want to honor what names each of them have chosen for themselves.

As a side, I work at a college and we have a small road race for students and families as part of our family weekend events. One year a student and his parents all ran the race, and when the parents crossed the line I didn't know their names but knew they were the parents of this student so I wrote down "Z's Dad" and "Z's Mom". When I went to ask them their names for the results list, I asked the mom her name and she told me her first name and then I asked "[Z's last name]?" and she said, well no, I didn't change my name, but that's fine. I told her "No, I'm getting married and not changing my name, what's your name?" and we had a complete bonding moment over it. She so appreciated that I cared about HER name.

1) correct

I think they would be annoyed if you used the man’s last name for a couple who doesn’t share a last name. I would do Mr John Doe & Mrs Mary William

2) For unmarried couple living together,

I would do Mr John Doe & Ms Mary William (on the same line)

3) Correct

4) I would send an invitation to each of them separately. 1 for the parents, 1 for the one adult “child” and her SO, and then 1 for each of the other adult children