のなら: emphasis or hearsay?

Question

What is the additional nuance in meaning when の is inserted before なら? For example:

トムが行く(の)なら、私も行きます。

In the Tobira Gateway to Advanced Japanese (p. 69) it says

の occurs before なら when the supposition is based on what the speaker has heard from someone or learned from the situation.

However, other websites such as https://maggiesensei.com/2015/11/07/how-to-use-%E3%81%AA%E3%82%89-nara/ and https://jlptsensei.com/learn-japanese-grammar/%E3%81%AA%E3%82%89-nara-meaning/ explain that の adds emphasis.

Answer 1

This の is an explanatory-の. Roughly speaking, it adds the nuance of "(if) ... is the case" or "(if) it's that ...". This の is usually optional, but it indicates the conditional is something that does matter now.

You can use の to seek clarification in an interrogative sentence (see this). This connotation is preserved when it's used with なら. If you understand the difference between "行く？" and "行くの？", you know the difference between 行くなら and 行くのなら, too. This type of の is often used with external information, but の is not a hearsay marker per se. It's not a plain emphasis marker that turns "if Tom comes" to "if Tom does come", either.

With that said, it's okay to add の unconditionally most of the time in conversations. The difference is usually very small. の tends not to be used with "generic fact" or "purely hypothetical" conditionals that have little to do with the current situation at hand (e.g., コインを投げて表が出るなら, 明日地球がなくなるなら, サンタクロースが実在するなら, ...).

Answer 2

The biggest difference for me is that の makes it a bit harder to pronounce when I speak and sticks out a bit in my head when I hear it. This difference goes away when の is reduced to ん.

I would say the following two sentences are practically interchangeable. I feel no particular emphasis in one over the other.

トムが行くなら、私も行きます。

トムが行くんなら、私も行きます。

なら could be rephrased using ば but の is mandatory in this case.

x トムが行けば、私も行きます。

トムが行くの／んであれば、私も行きます。

I understand this as meaning the function of の is already included in なら. You are already assuming a particular situation and describing that situation as you would when you use の.

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[Edit]

If I had to nitpick a difference, I would say the version with の gives an impression that the described situation is already decided outside of the speaker’s control, as if to say:

トムが行く（ことになっている）のなら、私も行きます。
If (it is already decided that) Tom goes, I will go, too.

The textbook is probably referring to this feeling of the information coming from an external source, as opposed to the speaker coming up with their own assumption.

But again, you could say the same thing without の.

トムが行くことになっているなら、私も行きます。

So I would say the difference is mostly negligible.