Hi. I think we can also use さながらの in the underlined part. But I'm not sure about the さながらに. Does the の (if used) modify 剣呑 while the に modify であった?
Thank you.
While you're correct that the difference between に and の would indeed be that the former connects to a predicate and the latter to a noun, 剣呑 is a な-adjective, not a noun, so they would not be interchangeable here. (If it were 猛禽さながらの剣呑さ, or the like, that would be a different story.)
In a case where you have a noun and a predicate to connect to, a situation like the one you describe could occur, like in the following example sentence from this page:
Well, I'm not just trying to discredit or disparage your dictionary, but if you don't tell me what dictionary it is or cite any specific examples it gives showing 剣呑 used as a regular noun, there's really nothing I can say, right?
Without any specific evidence to the contrary, I'm inclined to trust the Japanese-Japanese dictionaries that specifically list the word as a 形容動詞 with 剣呑さ being the related noun form. (Which is the only usage that I have personally encountered the word as well, though again, I'm not a native speaker, so perhaps my experience is limited.)