almost all phone and web shops are for companies based in the US. Can someone residing in Canada complete these shops?

I would say so. Make sure you have a long distance plan and pull up Google maps to be familiar with the locale. Double check with the scheduler. I have not done US shops before but I have done cross Canada.

Yes, they should be fine to complete from Canada, just check the shop Guidelines to see if it's allowed in the case of the specific shop, and you can always contact the MSP that posted the shop and ask, just to be sure.

Cheers!