In Polish it would not be Mrs/Ms Markowski or Janowski but Ms.Markowska or Ms.Janowska. The "j" is pronounced in English as "y" and the "w" as "v". The stress is on the first syllable.
Janowski is a common last name in Poland because it denotes that your ancestors came from a village/town called Janów. There are many villages called Janów in Poland and they are sown all over. Jan is the Polish form of the English name John. Literally "Janowski" or "Jankowski" is similar to the English last name "Johnson" as it denotes the same. The Russian equivalent would be Ivanov.
Markowski is also a generally Slavic lastname, judging by its spelling, meaning "descendant of Mark". There might be a couple reasons it was written Markowsky. It can be:
1.German transliteration of a Slavic lastname
2. Czech or Slovak lastname
3. a transliteration of a Serbian or Bulgarian lastname
Judging by the way you spell it "Markowski" I would say it is a Polish name.
Markowsky could be after an assimilated Pole/Czech who became German. Also take under consideration that the document writer was a German living in the USA. In Polish spelling, the last name suffix -ski is almost never ended with a -y.
Remember that Russian is not written in Roman letters but it Cyrillic. There exist many methods of transliteration of Cyrillic characters. I have mentioned some of them on the post about Bishop Yuriy/George. Also, in the past is was a common practice to transliterate words written in Latin alphabet to another Latin alphabet. example the city Kraków-(pronounced KRAK'UV), in Latin Cracovia, in German Krakau, in English Cracow.
Polish lastnames end in -ski/ska, -cki/cka (pronounced tski), -wicz, -ko, -iak, -ej, -aj, -ec, -ów(pronounced uv), -ch(pronounced "h").
The ending -witz is a German transliteration of the Slavic last name endings -wicz(pl),-vycz(ukr.), -vych(bel., ukr),-wych(ukr., bel),-wycz(ukr, bel),-vić(srb, cro, bosn, mn),-vich(all of the above). This ending tells us nothing more than that that the last name is Slavic.
You cannot delineate between Slavs based on their last names. Your heritage is that which your parents and grandparents passed onto you. There are plenty of Russians with Polish ancestry and Poles with Russian ancestry. During Tsarist times, many Poles were sent to Siberia, many Poles moved to Petersburg, Kiev-Kyiv, Minsk-Miensk and Moscow-Moskva for university and they eventually stayed and became Orthodox Russians. The same is with the Russian officials who were moved under the Tsarist regime to Poland, the Whites who found refuge in Poland, who also became Polish and Roman Catholic. Tracking down which generation assimilated into what other culture is hard to measure. The only criterion is the culture which your family adheres to. The majority of people here associate their nationality with the land that their ancestors are buried in and the state whose citizenship they have.