は vs が when the subject is modified by a relative clause

Question

Please note that the question is not whether the subject in a relative clause should be followed by は or が. (In that case I am aware that the thematic は cannot be used.)

I was reading section II.2, Wa and Ga, of the Structure of the Japanese Language by Kuno Susumu (1973). The section is a comprehensive discussion of those two particles and contrasted the use of the particles after the subject of a sentence/clause. Here is a sentence taken from the section.

Johnが好きな子はMaryです。

The author intended to illustrate the use of が in the relative clause. However, what I found interesting was the use of は after 子. I have two questions.

First, using は to mark the topic of a sentence requires some explicit or implicit context. In brief, my question is whether the relative clause can serve as context that enables the sentence "Johnが好きな子はMaryです" be placed at the beginning of a discourse.

When we say "as for X, ..." we need to make sure that the listener already knows what X refers to, either from previous context, or from general knowledge (e.g. X = the sun). The author wrote that the the thematic は requires the topic to be anaphoric, meaning that it has already been entered into registry of the present discourse. Obviously we can't begin speech with "子はMaryです" because the listener wouldn't know what 子 is being talked about. But when 子 is being modified with the relative clause "Johnが好きな", is that sufficient to make "Johnが好きな子はMaryです" the first sentence of a discourse?

Secondly, is there a situation that would make

Johnが好きな子がMaryです。

grammatically correct?

Edit: as noted in the comment section, it would sound very abrupt to begin speech with Johnが好きな子はMaryです. Let's pretend that the social dimension of communication can be separated from transmission of information. For the purpose of conveying information only, can a relative clause serve as context for the subject to be marked with は?

Answer 1

As you suggest, ジョンが好きな is enough for your listener to know you are going to talk about a specific child as opposed to children in general. However, that's not the reason は is used instead of が. Children in general can be a topic as in 子供は無料です whereas 子がメアリーです is no less weird as a standalone sentence than 子はメアリーです.

The question is whether the predicate is matching the topic. In your example, the predicate メアリーです clearly requires the topic (or topicalized subject) to be referring to one person. So the listener needs to know which one person you are referring to and the relative clause does help clarify that. If this condition is met, 子はメアリーです can also be completely natural.

妻の名前はナンシーです。子はメアリーです。

In general, a sentence with a nominal or adjectival predicate tends to sound neutral with は. This should not be surprising because when you talk about a characteristic or attribute of something or someone, you would expect your listener to know what or whom you are talking about.

There are situations where が sounds natural.

ジョンが好きな子がメアリーです。

This is an answer to who Mary is, rather than who the child John likes (or the child who likes John, depending on interpretation) is. It’s saying Mary is the one who John likes (or ditto).

The presence of a relative clause has little to do with which of は and が sounds more natural.

妻の名前はメアリーではありません。子がメアリーです。