Yes, it is a word. Adding the suffix "-ness" to most adjectives creates an uncountable abstract noun whose basic meaning is "state of being <adjective>". Dictionaries may not provide separate entries for these abstract nouns when they have not developed additional meanings besides "state of being <adjective>", but they are still real words.

Apparently so; scroll down: https://www.dictionary.com/browse/realness

Good question. This word is often used as slang. (gergo?) But it can also be considered a "real" word. The process of talking about the nature of reality often requires words that do not sound real, haha. Il processo di parlare della natura della realtà richiede spesso parole che non suonano reali, ahahaha.

I reckon it's got a slightly different nuance in LGBTQ+ slang.