Single spaces look odd to me, and yes, I'm in my 50s. But it does improve readability to me, breaks the thoughts apart just a bit. Not like we're still indenting every paragraph!
Whoa there, some folks still do. They're more accessible for dyslexic students, and also allow ASCII formatting for quick and easy questions creation on the fly.
I don't know much about that. I know monospace is used when quoting lines of code. I'm talking about general academic and professional writing, and especially scholarly writing.