Introducing an explicit subject into a sentence with a topic already existing

Question

Consider for instance the following sentence:

わかった、忠告ありがとう。でも、ミコトがどんな子かは自分で見定める。

The obvious context is that someone has been warned about ill behavior of his new friend by others but wants to make up his own mind.

The obvious implicit subject here is the speaker. However, the way I see it, both:

私はミコトがどんな子かは自分で見定める。

ミコトがどんな子かは私が自分で見定める。

Significantly alter the nuance of the sentence. The former would force the second topic to become contrastive, and a non-topical subject in this context has an entirely different nuance.

At least, I often see this pattern where a sentence seemingly “cheats” the rule of not allowing for more than one non-contrastive topic by making the second one implicit, almost as if Japanese actually allows multiple non-contrastive topics just fine, but not multiple explicit ones.

The only way I see to introduce it is with particle dropping:

私、ミコトがどんな子かは自分で見定める。

Which “traditional grammar” teaches is not appropriate in formal contexts, though common informally and is “technically” a grammatical mistake. But, I've actually seen particle dropping in this context specifically multiple times in sentences that otherwise eschewed informal grammar, suggesting that it is indeed the only way.

So what's going on here? Is it actually not possible in sentences such as this to introduce an explicit subject while keeping the nuance without resorting to particle dropping, or is my understanding wrong and does “私はミコトがどんな子かは自分で見定める。” not force the second 〜は to become contrastive? And if so, does this only applies to less common “〜かは” situations or also to say “その英語は好きじゃない。” vs “私はその映画は好きじゃない。” when someone is talking about a particular film.

Answer 1

The former would force the second topic to become contrastive

That’s not necessarily true. ミコトがどんな子かは is clearly shared information and the primary topic in this context whether there’s another topic. In other words, starting with 私は expresses what kind of person the speaker generally is and that sounds a bit abrupt. The contrastive interpretation doesn’t really make sense because there’s no information compared with the speaker, i.e. that about another person.

the rule of not allowing for more than one non-contrastive topic

There’s no such rule to begin with.

ミコトがどんな子かは私が自分で見定める

This 私が virtually means “instead of other people”.

The only way I see to introduce it is with particle dropping: 私、ミコトがどんな子かは自分で見定める。

I agree with you. Zero particle is a thing in terms of Japanese grammar, or rather, it’s what indicates the subject in classical Japanese, and still used today.

Answer 2

You can imagine this original sentence with no topic particle:

1. 私が自分でミコトがどんな子かを見定める。

In this context, this topic-less sentence is already perfectly natural because the exhaustive-listing sense of が ("It's no one but me who...") is suitable.

From Sentence 1, you can topicalize the subject, 私:

2. 私は自分でミコトがどんな子かを見定める。

The explicit exhaustive-listing nuance is now lost, but this still sounds natural in this context.

Alternatively, you can topicalize the object part (with no contrastive nuance):

3. 私が自分でミコトがどんな子かは見定める。

The original sentence is the same as this except that the word order is a bit different and that the subject (私が) has been omitted. So this is what's happening in your original sentence, and I don't think you have to "cheat" to parse it.

If both are marked with は:

4. 私は自分でミコトがどんな子かは見定める。

I'd say this sentence does start to convey a mild contrastive meaning around the second は, somewhat like "(At least) as for who Mikoto is, I'll decide it".

When a comma is used instead of the first は:

5. 私、自分でミコトがどんな子かは見定める。

This is a delicate issue, but personally, I feel it's somewhere between Sentences 3 and 4. In this example, it's completely fine to omit は like this since it's conversational. In formal written language, you should choose from Sentences 1 to 4 depending on the nuance.