STD Addressing question (different then Katie's)

I'm getting together the addresses for our STD's (which i ordered yesterday from magnetstreet and saved 27%. I'll post a pic when I can)

So yes, back to my question. Married couples I know I address as Mr. & Mrs. (insert husbands full name here) (last name)

Single females as Miss.
single adults as Mr.

Fine, that's all great.
BUT

How do I address them if a.) a guy and a girl are dating, not engaged, and both would still be invited if they break up? I assumed it would be.
Mr. John Doe
Miss. Jane Blue (first word that popped into my mind lol)

b) If they are dating but if they DO break up I would only invite one of them? But they've been dating for years so I have to include the girls name.

c) Should the guy's name ALWAYS go first? Even if out of the couple the GIRL is the direc invite?

Well first, any single females over 18, I'd do Ms., not Miss.

As for the other questions, I put the girl's name first on all of these, and I'd do the envelope to both people (even if you wouldn't invite one if they broke up). If they do break up, you deal with that if and when the time comes.

Sooooooo

Ms. Jane Doe
Mr. John Smith

for couples...i looked up the ettiquette on this ( i also agree with laura that you dont worry about if they break up until that occurs....so just follow ettiquette for now):

if they don't live together you put the name of the person whose address you are sending it to and then their significant other after.

if they do live together....you do their names alphabetically based on last names:

Mr. John Doe & Ms. Jane Smith

or

Ms. Jane Doe & Mr. John Smith