After finishing Hook Mountain Massacre, are we able to choose cards freely from decks B, C, 1, and 2 before starting Fortress of the Stone Giants?
This basically mean that we can get cards from current adventure deck - 2 without having to actually obtain them during the runs. Is this even possible/acceptable?
Not completely. What you can by the rules do, is if you don't have enough cards in your pool of ALL players at the end of a scenario, is go back and pick from B, C, or 2 adventures behind.
There are certain ways you can "help" this process along, but you can't just automatically chuck cards to get new cards.
If you are wanting to thin your spells, you could pass them to a character during the adventure without the Divine or Arcane trait and have them cast it so it becomes banished. Just remember that your total spells acquired have to be low, or this might backfire. If your group finds and acquires a bunch of Mendings, Guidance, Levitate... well, you get the point.
Let's say we run a scenario in deck 3, so I intentionally banish a weapon and throughout that scenario nobody got spare weapons. Then that's the time I can go get a weapon of my choice from decks B, C, and 1?
Isn't that just the long version of just picking out that weapon from the decks directly?
Ah yes, but there are scenarios where there are locations that requires you to banish a card. Lots actually. And there! Banished. But yeah, I do get the general idea on this. Just checking.
It's not easy, but it's doable.