Extracting from, and introducing to Universe of Discourse using は and が with こそあど-demonstratives

Question

Consider the following:

1. このりんご（ は / が ）食べないでください。

2. Person A: どちらが東ですか。

   　Person B：　こちら（ は / が ）東です。

Workbook answers are は for 1. and が for 2.

• In 1., I would expect the person to be pointing at the apple or at least gesturing with body language towards the apple. Furthermore the apple is physically present within proximity of the speaker as evidenced by use of "この". Therefore the apple is already present in the "universe of discourse" and the use of は to extract it from the "universe of discourse" is justified.

• In 2., Discourse is introduced by person A. The workbook chooses が as the correct answer.

(Observations) The apple is tangible while direction is intangible. However, both are present in the "universe of discourse". "Direction" is part of "immediate environment" and I should always be able to refer to immediate environment using は since it's shared information between two parties.

(Question) Can't I similarly take "direction" as already present in the "universe of discourse" and use は to refer to it in 2.?

EDIT: Concept of "Universe of Discourse" taken from jkerian's answer

Answer 1

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 possible to make this a topic rather than こちら:

東の方向はこちらです。