I've read your explanation, of course. The reason why I asked is that it looks as if there isn't only one way to rephrase the sentence ('Whoever took them'). Having made a question from it, it became obvious to me that 'Whoever took them' is a clause. Moreover, in the grammar book I mentioned in the first post, the author called it a clause. And it was exactly with 'whoever' (not 'who'). So it also looks as if there isn't only one opinion about this sentence and the underlined part in it. I remember Piscean also called it a clause in post 2.
Secondly, I don't see any cardinal difference between the two sentences ("Who took them" and "Whoever took them"). There might be a slight change in meaning, but not that big, as I see it. (Historically 'whoever' could have been two separate words. Just a guess.) I wonder if Piscean saying that the sentence with 'whoever' could be rephrased with just 'who' instead (in post 25) sees a big difference in meaning.