Going from memory, I believe the S8 is the first phone to offer the newest Fast Charging spec. So the old docks should still do fast charging, just not as fast as the brand new fast chargers built to the new spec.
No, that's not what he's asking. Re-read his post. He asks if he chooses to return the phone within the 15/30 rule can he also walk away from the contract and go with another provider. The answer is yes, he can.