Going into Crimsonwood Keep

Do we need the Keystone? If so, where do we use it?

It's required to start the Masteria PQ.

But not to actually go into the Keep, right? And where is the entrance to Masteria PQ?

Go to the warp @ the Crimson Guardian map (it's on the door)
Walk left a bit, enter the portal.
Climb to the top left.
Talk to the lightning gate, it'll automatically take you to the next room.
That's where Barricade is, and you start the PQ (note that this room is channel-change blocked)