That was my first idea as well, but I could not really fit it into the sentence…
Alright, since you asked for it here is my thought process:
A few lines earlier, the boy speculated
血が飛ぶ…? - the blood is flying?
to what Kiki echoed
何が飛ぶ… [?] - what is flying [?] (the question mark is from the かしら, so to speak)
So my exhibit A is that this is a proper sentence.
Now let’s add the の and let’s assume it is a nominalizer. What would this nominalizer attach to? The closest part of the sentence would be 飛ぶ, like you suggested, which would make 飛ぶの into a noun (“the flying”), also like you suggested. Now we end up having our sentence as:
何が (飛ぶの) - what is the flying?? I have a hard time trying to grasp the meaning.
Also, I’d rather expect a verb after 何が instead of a noun (but that might just be because I’m more used to seeing it that way).
OK, next attempt: の nominalizes the whole sentence, as in
(何が飛ぶ)の - the “what is flying” - still feels strange but I could agree with this more easily.
Now let’s add かしら (luckily it is possible to attach it to a noun, as shown in this DoJG example):
((何が飛ぶ)の)かしら - following the DoJG “teacher” example, the translation would be something like
“I wonder whether sth. is the ‘what is flying’” - but we don’t wonder whether something is it, we know something is it, and we wonder what it is, so that does not work out for me understanding-wise either.
OK, maybe it is not the nominalizing の? What else could it be?
Possessive? Clearly not.
Pronoun? Doubtful.
So, last not least we have the explanatory の:
When I looked into this yesterday, I found this option doubtful as well, but now I think it might be the best option tbh
What do you think, @Shannon-8 ?