I’m not sure as to sources for the halacha, but as for question #3, my father was told the following by R’ Dovid Cohen: you take maaser after you finished paying for your basic needs. So, take your income, pay your rent/mortgage, food, clothing, etc. and take maaser off of what’s left.
I’m sure there are varying opinions on this, so ask your Rav what he paskens on it.