A giant YOU to anyone who answers one of the first four.
Until the US Canada gets its act together when it comes to gun control (i.e. never hopefully), the borders should not be an open free for all.
Why should Canada's gun laws matter to the US?
The illegal importation of guns into Canada is a problem, and would be worsened with fewer border restrictions. I don't see how our strict gun laws in any way impede the US and would further impede the US with more open borders.
The U.S. should not admit new States unless those new States adopt the bill of rights. Some rights transcend borders, and the U.S. should be suspect of integrating with any countries, especially those which burden those rights. I'd say the same thing if Canada did not have speech protections or sufficient criminal due process.