Basically I’m wondering if です can be nominalized.
です is not the dictionary form.
Dictionary form for です is だ.
So, your question should be asked as:
Can だ be nominalised?
Nominalisation (or to nominalise) litteraly means [to make a noun].
So, if to say your question in simple words, than it would turn to:
Can I make a noun from だ?
If to look at how ...
i-adjective + である is not grammatical for the same reason i-adjective + だ is not grammatical: i-adjectives already serve as a predicate without need for a copula (or in other words, you can imagine that the meaning “is”/“to be” is embedded in the i-adjective). i-adjective + です is grammatical, but the です here is not the usual copula, it is just a polite marker ...