Ulala, that's quite a hardcore level of grammar, it's a participle. As hover tips are general translations that could be possible in some situations, that's shown here, but absolutely won't suit this sentence.

Basically, the participle of that kind would be used in a similar sentence: I see a man, drinking coffee = Widzę mężczyznę pijącego kawę. "pijącego" is here Accusative from "pijący" - the participle 'drinking'.

Therefore 'będącego' is the Accusative participle from 'being'. Such a word won't be used too often, but let's try and imagine something, however that won't be very natural with this specific one: I see a man, that is/who is/being my father = Widzę mężczyznę będącego moim ojcem/tatą. You will encounter participles later in this tree.

This is a question that I find more and more often. The problem is, that you treat the construction of Present Continous too literally. If you had Present Simple, it would be clear: "drinks" = "pije", no place for doubt. "is drinking" is just a grammatical construction. It doesn't really add another verb (to be) to the sentence. Not in Polish, definitely. Present Simple and Present Continous are translated the same way in 99% of the cases. Your sentence sounds kinda like "Who is drinks coffee?" It just doesn't make sense, because you added an additional verb to it.

Generally yes, but that's a question about the identity of some specific person who is drinking coffee. "Who is this person over there, the one drinking coffee?"