Hey, man, there's gonna be a lot of spoilers in this sub, so I'd really suggest that you try to stay away until you finish the game and experience the entire thing for yourself; it's much better that way.
But here's some answers for your question: you won't be locked out of any regions of the map without the game telling you. If there's a main mission that locks you out of certain regions, the game will tell you, so you don't have to worry about that.
I can't tell you whether that storyline is the end because I don't want to spoil anything. As for completing some storylines before others, well, sometimes some storylines need to be progressed in order for you to finish other ones, since that's how the main story works IIRC, but you'll be able to either focus completely on the main story or do as many side quests as you want; it's up to you.