Actually modern Polish language slowly goes towards trying to create new feminine forms where there used to be only a masculine one, so such a basic word as "pisarka" is definitely a necessity ;)
Nowadays „pisarz” means only „author of books” (usually romans).
In the old days when the literacy was much lower it was an occupation. Also some officials were entitled „pisarz” like „pisarz wielki koronny” (łac. notarius magnus). In modern Poland this kind of job is known as „notariusz” (notary)
No. Unless they write a book, perhaps. But still, there is a separate word for a journalist: dziennikarz/dziennikarka, so even then he/she would probably be rather treated as a journalist who wrote a book.