Always address people using the names, titles and forms that they prefer and that they themselves use. A lady who aspires to be a proper hostess should start keeping track of those things in her visiting-book or iCloud contacts list early in the planning process … say, about three years before she meets her future intended. If you have dropped the ball on that, then you may have to ask people how they prefer to be addressed.
If it were 1970 or earlier, and for some reason you had not been keeping track of such things, then you could resort to standard etiquette for an easy answer to your questions:
1. Divorced, kept married name (regardless of whether she is engaged to anyone or not): Mrs Jones Smith (where “Jones” was her maiden name.)
2. Divorced, changed back to maiden name: Miss Jane Jones (just as if she had never been married.)
3. Divorced, kept married name…now back together and living with ex-husband: Mr and Mrs John Smith (people didn’t live together openly unless they were married, so you treated them as married)
4. Widowed ladies keep their married name and title: Mrs John Smith
Post 1970, the old title “Ms” was re-introduced as a feminine equivalent of “Mr”: to be used for all women regardless of marital status, with the woman’s own given name. This is the current standard default for when the lady’s preference is not known:
1. Divorced, kept married name (regardless of whether she is engaged to anyone or not): Ms Jane Smith
2. Divorced, changed back to maiden name: Ms Jane Jones
3. Divorced, kept married name…now back together and living with ex-husband: Mr John Smith and Ms Jane Smith
4. Widowed ladies: Ms Jane Smith
Isn’t that lovely and simple? But “Ms” was also the marker for a second-wave feminist, and was rejected completely by traditionalists and by backlash anti-feminists, who got all insulted to be addressed as “Ms”. And young women in the process of getting married are often as excited to wear the title “Mrs” as to wear their diamond engagement set — whether they change their name or not; and are appalled at the idea of changing their first name for business purposes, even when they don’t think twice about changing their surname. “Ms” has made it into use as a standard business title, and has replaced “Miss” for unmarried women over 18 and under, oh 60 or so; and has become common for divorcees who are no longer so enamoured of the thought of being a “Mrs”. So, even before Mrs Michelle Obama blew away the etiquette ruling that “A lady’s given name is NEVER used with the title Mrs”, the most commonly-used form that would be offensive to the smallest number of people, was and is now:
4. Widowed ladies: Mrs Jane Smith
5. Happily married couples:Mr John Smith and Mrs Jane Smith
5. Happily cohabiting not-legally-married couples: Mr John Smith and Ms Jane Jones
And since one reason for etiquette is to avoid offending people, those forms are a pretty good guess. Not the formally-correct protocol-approved guess, but still probably your second-best choice if you cannot simply get the correct forms right from the source.