Unknown name of mother
Not sure of the "proper" way to handle this situation: I know the father's name but not the mother's. As far as I know, they were married, but I have no source to verify that. The only thing I know about the mother is where she was born (from multiple census records of the child). Would it be advisable (or inadvisable?) to enter an unknown name placeholder ("?" or "Unknown" or "Mrs....") in order to enter the mother's birthplace? Any opinions or guidance is welcomed. Thanks.
If you don't know the name, then the proper practice, according to the FSFT guidelines, is to enter no name at all. With the father's name, which you have, you can still connect all the children. And, perhaps once you have connected one or more children, a record containing the name of the mother may present.
Entering terms such as "unknown" or "wife of" or "Mrs X" does not help other find the family.2
As Áine says, if you don't know any part of a person's name, it's best not to create a profile for that person. You could create a custom fact on her husband's profile, titled something like "Wife's Birthplace", and tag it with the census records and other sources that it derives from.2