From my experience, it seems that Mrs. is becoming archaic, as Ms. is used instead. Anytime I refer to a woman with a title other than Dr. (or Hon., Judge, Prof., etc.), even when I know she is married, I normally use Ms. Almost all legal documents that I encounter use Ms. (although that doesn’t mean that the lawyers drafting such documents don’t make glaring grammatical errors
).
Of course, some married or widowed women prefer to be referred to as Mrs., in which case that form is used.
I guess the professional usage of Ms. is based in part on the fact that sometimes one doesn’t know the marital status of the woman, as well as the fact that usually a woman’s marital status isn’t relevant in the matter. Further promoting the use of Ms. over Mrs. is that many women keep their surname after marriage, as saying “Mrs. X” implies that there is a “Mr. X”, while in fact he might be Mr. Y (although given Western patriarchal tradition, I’m sure many people have accidentally referred to Ms. X’s husband as Mr. X, even if he’s actually Mr. Y).
On the other hand, in some matters involving marital status, using Mrs. might make sense (for instance, in my line of work, tenant succession rights can be based on marital status). Even so, I only use Mrs. when the Mrs. refers to herself as such beforehand.
Are there circumstances in which a non-married or -widowed woman is referred to as Mrs.? I’m assuming in a D/s context such abbreviating isn’t allowed and is strictly punished 
I use Mr., Dr. and Ms. fairly regularly and don’t abbreviate them, but I never thought that using the long forms was incorrect.
Anytime I use an acronym (other than in informal writing such as message boards
), I spell out the word first and then put the acronym in brackets. Of course, I only do this when I know that I will be using the acronym again, and do not use the long form after the introduction of the acronym.