Other hero missions after beating the game.

Ok so here is what happned.
I chose Master of Whispers and that got me to do Dzagonur Bastion.
After that on the same night I was with a guildie that picked Margrid the Sly and was able to do Dasha Vestibule.
I beat the game and I have the quests to get the remaining heros.

My question is,
If I had not done Dasha Vestibule then how would I actually do it.
It wont be on my map and I would have never known that it exists.
If I pick up Margrid the Sly after beating the game, would I get another quest telling me about Dasha Vestibule?

No, to do the two Margrid missions, Pogahn Passage and Dasha Vestibule, you just need to get to the outposts and have Margrid with you and you can start the missions without any quest requirements. The only "hard" part is finding the Dasha vestibule entrance without the quests, but it will open near that Res Shrine just northwest from Honur Hill.

I took the village mission from the princes (not the gardens one) and went back to the bazaar after completing the game and there was teh Learner guy there who gave me the mission to go the gardens. I assume it works like that for every split in the storyline the person who gave the quest will give the other one...

I also grabbed all the heroes I missed in the area after completing the final mission, then it was 3 map travels away!