is the correct parsing. Here,
なにか is a noun, meaning
something. In kanji, it is「何か」.
なの is actually a composition of
の, where ..
だ is the same
だ as in 「今日はいい日だ。」「明日は休みだ。」
の wraps the preceding sentence as a noun clause. To connect to this
だ is conjugated to
だろう indicates that the speaker/author guesses so.
The structure of the sentence,
"I guess （だろう）definitely （きっと） that
By the way,
それ points to
これ at the beginning of the text you cited.
The essential part of this sentence,
has the same structure as, for example, this sentence,
with the correspondence of
なのだろう is used to guess the reason or cause of some observation.
For example, you can use it like
この若者はとても酔っている。明日は休みなのだろう。(This young man is so drunk. I guess he must have a day off tomorrow.)
(A middle aged man is running up the stairs to the platform of a station. I guess this train must be the last one.)
そのネズミはずっと震えている。病気か何かなのだろう。(The mouse is shaking all the time. I guess it must have a disease or something.)
From the perspective of those people and animal, the situations are like,
- とても酔ってしまった。でもだいじょうぶ。明日は休みだ。(I'm so drunk. But that's O.K. Tomorrow is a holiday.)
- 急げ。これが最終電車だ。(Hurry up. This is the last train.)
- 震えが止まらない。病気か何かだ。(I can't stop shaking. I must have got a disease or something.)
なにかな, meaning 'What is it?', is decomposed to
なに: pronoun of question
か: a particle indicating question
な: a particle of some emotion
This set of words may have some common origin with the set of words in
なにか+だ（な）+の+だろう (especially なにか(something) should have been derived from なに(what)), but as phrases, their meanings are distinct from each other.
The translation 'marveling' should have indeed been chosen because of the stressing phrase,
I am eiditting this answer to respond to the comment by Mauro at Oct 6 at 7:14.
How can you say that それ refers to これ?
I said so by analyzing the meaning of the text you cited.
The first sentence tells that
これ is a mental illness (according to the guess of the author) (and hence that the author and his/her mother stop some celebrations.).
これ could not be named or diagnosed clearly.
The second sentence adds more explanation about this unnamed something like an illness --- the author tells what kind of thing he/she guesses it must be. If you agree with this interpretation of the sentences, the subject of the second sentence
それ must be pointing to the unnamed something like an illness, which was pointed by
これ in the first sentence.
Also, in his reply naruto said の in an explanatory, while you say is a nominalizer; as far as I know だろう can be attached to a noun, so I don't really see the need for a nominalizer, and I was wondering about this difference in your and narutos' answers.
The link provided by naruto,
is very good. As the linked answer says,
の is a formal noun (*1) which can be translated as 'a thing', 'a fact', 'a case', etc. Therefore, from the perspective of synatx, this
の is what you call a "normalizer".
However, from the perspective of semantics, this
の is used to load some meaning to the sentence, and hence it is "explanatory".
In other words, as you thought,
are both valid expressions. However, the former expression adds slight emphasis to the statement. The linked answer explains the mechanism how such emphasis arises by
の. Also, I would choose the former expression over the latter instinctively for the particular sentence in the text you cited. Perhaps the nuance of 'filling in missed information' attached to
の, as explained in the linked answer, might be suitable for expressing a guess, but I don't know.
*1. I think grammatical classification of
の is a particle (格助詞), rather than a noun, which can be used in place of a noun.