You absolutely don't need a data plan - my girlfriend is on Rogers with a smartphone kitted out as a dumbphone. Just turn off packet data (in the network settings menu) and you're set. You might not be eligible for subsidies on some devices though: many carriers have a minimum monthly cost to get the full discount.

Almost always, yes, as long as you own the phone. I had a non-data plan for years with FIDO. If the rep says you can't, ask for someone else.