Assuming it is full ATC (you say Tower and you say you were asked to join left downwind) then I would assume he was clearing you to join downwind for a left hand circuit whilst a right hand circuit was generally in use. There could be many reasons for this, the most obvious being that you were approaching from that direction. It’s not at all unusual, so just read back...”Roger G-xxxx to join left hand downwind”. He will soon correct you if it’s wrong.
Yes, this.
It is totally fair and wise of you to ask for clarification/confirmation if you don't understand. Once a tower instruction is clear, follow it. It is a very wise habit to read back the critical element of a clearance. Not necessarily required for VFR flying, but very wise. Because, if by chance, what you read back is what you understood, but not what the controller said, it then becomes the controller's responsibility to sort it out. Read back what you understand to do, then do what you read back.