In 2., こちら is the focus, not a topic, and is new information. You cannot use は for it. 東です is the old information. Your reasoning of "present in the universe of discourse" is irrelevant for the choice of these particles (and, actually, I don't understand what you are talking about). The information structure is the following.
After A's question, before B's response, the following is the old information shared by A and B:
Old Information: There is some direction that fills in the underline in:
__________が東です。
B's response gives new information, that is, what fills in the underline. The new information provided by B's response is this:
New Information: It is
こちらthat fills in the previous underline.
Since 東です is the old information, it is rather possible to make this a topic rather than こちら:
東の方向はこちらです。