Yes, most Russian surnames have gender variants. Not all of them though.
Yes. Unlike Lithuanian, in Russian, there is no distinction between married and unmarried women’s surnames.
The surname is OK, but the patronymic from the name Илья should be Ильинична (pronounced like /ильйи́нишна/). An irregular one. Masculine form is Ильич /ильйи́ч/.
First names decline like nouns.
Your example is a bit amusing though since there is also a female surname Марина which declines differently (and also has a different stress)
first name last name nom, Мари́на Ма́рина gen. Мари́ны Ма́риной dat. Мари́не Ма́риной acc. Мари́ну Ма́рину instr. Мари́ной Ма́риной prep. Мари́не Ма́риной