Isn't it possible to still call them Knopf too? Like how buttons on a keyboard would be "key" in English. Similarly in Swedish, a general button is "knapp" (related to Knopf) and a button on a keyboard is often called "tangent" (not related to Taste), but you can still call them by "knapp" too.
Yes. Some verbs require the nominative after it.
"The nominative case is also used to describe predicate nouns (i.e., nouns that are on the other side of verbs such as sein, bleiben, heißen, werden, scheinen from the main subject)"
Taken from here: http://coerll.utexas.edu/gg/gr/cas_02.html