Can it be used to answer a question. I.e. "would you like to go for coffee?" "Why not!"
If "var" means "what" and "för" means "for", then "varför" could literally mean, "what for". "What for?" is an informal way of saying "why?" in American English. I'm curious, do other English-speakers use "what for"?
O Romeo! Varför är du Romeo?
Förneka far och namn: om du ej vill det,
Så svär blott att du älskar mig, och jag
Ej längre vara vill en Capulet.
Except of course she was not looking for him, she was asking why he has to be Romeo Montague, because as was just established, "wherefore" means "why", not "where".
Sometimes. It depends on the context of the situation and region of the English speaker. Someone could say, "I have to see my boss" and their friend could reply, "What for?" instead using the word 'why', but it's not really that common.
You would actually use "Varför?" to answer that question in Swedish however, or at least it's a possible response.
Not sure what exactly you're asking, but that would mean either "Why not, then?" or just be a more casual way of asking the question.