i want to ask those who completed the game,will i get the opportunity to do witcher contract or free roam after act 2 'Battle of Kaer morhen anytime in Main story but before the completion of main story ?

Not any time, no. Once you've completed Bald Mountain with Ciri you move into 'Final Preparations'. You are free to move around the world then, but don't talk to Avalla’ch until you're ready for the big finish.

so i can finish witcher contracts & few side quests at that time anywhere on the map?

Yes.

thanks