Dear Jill,
Jewish is an ethnicity and a religion. It is not mutually exclusive that the family was of Jewish origin but of Christian religion. Many Jews converted to save themselves from persecution or simply because they wanted to. And yes, there were Jews from Minsk and Belarus. Jews also moved around a lot and hid their identities if they needed to, especially in places in Russia where they were not supposed to be found.
If you're searching for ethnic origins, you may find the Jewish website more helpful. And since you haven't had much success elsewhere, you might find different and interesting results there.
If Theodosia is talking about her father-in-law Moisey's baptism, he likely wasn't baptised as an infant.
By the way, Galicia, Galitsia had a very large Jewish population.
Tamara farache