You're right. というのである＝というのだ。 のだ/のである are kinds of expression of a sentence end. The function is to indicate that the sentence plays a role such as explanation, reason, or summary for the former sentence by emphasizing it. It's an explanation and the reason to 不審に思う. And The first sentence starts from まだ十香～～is the first reason. In this sentence, it means that it's narrated by a third party like a narrator who is explaining the situation.

I guess というのである has two ways to use. First, "something というのである" means "someone says something." Second, it is used as just a literary expression. It is enough in our daily life to say "現れた。". However, if an author wants to make his novel more unique and authentic, he sometimes change the end of sentence like this. Therefore, if you can ignore this part, というのである, it doesn't matter. I also guess that this phrase is sometimes used in the case that something negative happens in a row. For example, 昨晩彼女にふられた上に、今朝の試験に遅刻したというのである。彼が落ち込んでいるのも無理はない。This means he was dumped by his girl friend last night and he was late for the exam this morning, It is understandable that he is very disappointed now.