Why would it not be? You can form "open-mindedness" from "open minded" the same you can form "yellowness" from "yellow" or "coldness" from "cold". It's a valid way to make a word from almost any adjective. Syntactically it's weird because unlike most other Germanic languages English likes to put spaces into compound words. But that hasn't stopped open-mindedness from catching on and getting mentioned in dictionaries.
It seems so. The Wiktionary entry [1] has existed since 2007 and lists translations to several languages. The French one is quite common, it makes sense that English has a counterpart.
It's English. Any verb can be nouned, and you can verb any noun, and people will understand you just fine. It's a feature, perhaps more cultural than linguistic, but a feature nonetheless.
have you considered that you may hold the opposite attribute?
I suppose the audience here never watched The Office.
You've never heard that phrase? Its pretty common.