You should be able to enter Canada as a visa exempt visitor - say, you cross the border, they ask what the purpose of your visit is, it's to visit your spouse. You don't have to give them more than they ask for and it's pretty normal to be an American married to/in a relationship with a Canadian. From inside Canada, you can apply for an extended one-year visitor visa. I am not sure exactly what the stipulations of this are as I've never had one. This will allow you to have status for longer than your regular status as a US citizen visiting Canada (which is six months).
You can apply for inland sponsorship as soon as you've arrived in Canada. It's not illegal or wrong to do so. It would just be prudent for you to make sure your status doesn't run out. There are lots of options. Applying inland can allow you to apply for an open work permit, but these vary in the length of time they take (2-4 months from when you apply) so it would be smart to have the visitor visa extension ready for when your six months run out, even if you go the open work permit route.
I don't know what would happen if you tried applying for a one-year visa as an American from outside Canada. It seems weird for them to reject it as you'd be trying to do things as legally and smoothly as possible. But you are completely free to cross the border as a US citizen, as long as you don't have any other issues that would prevent you.
