Yes. "Ms." is not the same thing as "Miss", but includes both "Miss" and "Mrs". In Finland, neiti ("Miss") is dead as a title. It's only used to address little girls. Rouva is used as a title if such a title is for some reason needed. When the former president Tarja Halonen took office, she was unmarried and was referred to as Rouva Presidentti. :)
Yeah, 'rouva' is closest to mrs., but can also be used to refer to an "older" lady whether they're married or not [i.e. ma'am].
'Neiti' is generally a girl or a young lady (maybe between teenage years and 30's), but 'neiti-ihminen' could be used of a woman regardless of age if one wants to specify that they're unmarried (especially of those who've never been married).
However, using these as titles is very formal and thus not very common anymore.
Exactly. We are using the abbreviation "Ms." in most (but not all) of the sentences as the main translation, since it says nothing about marital status. We accept Mrs. as well since that is the original meaning of the word rouva. "Miss", however, is not accepted. About neiti: it refers only to children these days. If you use it to address an adult woman (especially if you use the word without any other word, like last name, attached to it), some may consider you rude.
My Finnish friends told me the word "rouva" does indicate marital status (married). If that is the case, then "mrs." would be the proper translation in almost all cases, when the translation goes from Finnish to English.
Above is an example with rouva being used as a title for an unmarried president. There's a similar construction in Swedish for the chairman of the parliament (riksdag): "Fru talman", which would be "mrs. Chairman", no matter marital status. Also the female judges of the High Court of Justice of England are titled Mrs. Justice, regardless of marital status. Isn't Rouva Presidentti just such a rare fixed title, just like these Swedish and English examples?
My point is: I don't see why you don't put it the other way around, with "Mrs." as the main translation, accepting "ms." as a second choice. The current main English translation "ms." opens up for the possibility that rouva Pöllönen is not necessarily married.
If you need to be formal, rouva is the word to use regardless of marital status, even though the original meaning was strictly "Mrs.". Or then you can use some other title to replace it, like professori. neiti is no longer used as a title, except as a joke. It only appears when addressing young girls. Nurses and doctors in child health centres are particularly fond of it: Aino on iloinen nuori neiti, "Aino is a jolly, young miss". It is true though that if you are using titles, you are more likely to use rouva in fixed titles like the ones you list, and of married women. Finns are even less formal than Swedes.
"Ms." is not an abbreviation of "Miss" but of "Mistress" and does not specify marital status. In fact, "Miss" is already an abbreviation, of the same word, "Mistress", but with a different function. "Ms." is pronounced /ˈmɪz/, whereas "Miss" is pronounced /ˈmɪs/. Have you ever read Fever Pitch by Nick Hornby? Or seen the British film version (starring Colin Firth and Ruth Gemmell)? There's a bit in the film in which the female teacher played by Ruth Gemmell introduces herself to a new class and makes sure that the kids know the difference. Go to Wikipedia to read more about "Ms.", its function, and its history. :)