Post # 1
There are so many rules for addressing the envelopes of the invitations, is it the same with thank you cards?
Should I address the envelope to Mr. and Mrs. John Doe or can I be more casual and address the envelope John and Jane Doe?
Post # 3
We actually addressed our invitations as "Jane and John Doe" and our thank you notes as well. The very formal rules are technically applicable for any formal event – so unless you’re asking your guests to wear black tie, you don’t really need to follow that convention.
The thank you note is like any other card you might send – a Christmas card, for example. So if you would normally address your Christmas cards in the formal manner, I would do that for the thank you notes as well. If not, then go with the less formal convention.
And, if you’re going to use first and last names rather than titles, the correct usage is to provide the woman’s name first (Jane and John Doe, rather than John and Jane Doe). Inside the thank you note, you should order the names (Dear Jane and John) according to which person you are closer to – so if you’re writing the note, and John is your cousin, it would be "Dear John and Jane," whereas if Jane is your friend and John her husband, it would be "Dear Jane and John". Same for any notes your Darling Husband writes. If you’re equally close to both persons, then again the woman’s name goes first.