1. Realtor fee is paid by the tenant in any transaction that takes place on a Realtor.ca or MLS listing.
2. Not necessarily. But a landlord that is hiring a Realtor (and paying them) and asking the Tenant's Realtor to bring a qualified Tenant (and is agreeing to pay them too) obviously sees value in having a Realtor. They are professional (mostly, haha) and the transaction is secure. If the Landlord wants to use a Realtor, they hire them on principle, not on saving few fees. It doesn't affect the rent price or make it more expensive.
3. Check online, read reviews, go through websites and social media. Realtor.ca has listing of all licensed Realtors in Ontario.
Good luck!