Not necessarily. Commas are a feature of sentence structure.
No, before ... as in: Marcus Welby, MD
No, a comma does not go before the word 'in'.
Yes. You should write it at John P. Doe, MD
Not necessarily. There is no word in English that requires a comma before or after it.
no
No.
after
After
Before
It could go either ways?
Yes, but a space comes after the comma like this:, inc