As a Canadian, I appreciate your thoughtfulness in asking these questions. Thank you!
1. Either is acceptable and appreciated.
2. Absolutely!
3. In a major centre like Vancouver, it's pretty easy to exchange US$, any bank will do it.
4. Not at all offensive.
RE: Airport. When you leave Canada, if you have a bunch of Canadian $1 and $2 coins, use up as many of those that you can. You can't exchange them back to US $s in the US.