If I marry a Canadian Citizen, will it make any difference at all to my US Citizenship and Residency? Will it make any difference at all to staying in and leaving Canada? Will it make any difference at all to staying in and leaving the United States?
No, you cannot lose US citizenship simply by marrying somebody else (no matter who it is). Your right to US residency is linked to your US citizenship, so you always have a right to enter, live, and work in the US.
Similar for Canada, as a Canadian citizen you always have a right to enter, live, and work in Canada.
Your future spouse will find it easier to live and work in the US because of your US citizenship. There's still some paperwork, but it's simpler.