There is no default tone. Even if there was, you might have problems. Because meanings are different for different tones.
Yes. That's true. Many of my friends use 刀 as well. They even use "人刀" for RMB.
I don't know a single word. But the idea is "challenge an impossible mission". You may come up with a right word.
Well. I normally say： 凑合吧/还行吧/差不多吧/马马虎虎吧。哪里 is a formal word. It is OK to use in a formal case. But in the daily life, it it not too formal but too nerdy to use. E.g. A: 你今天很帅气! B: 马马虎虎吧