Yes. In a way they both have power. If Remus decides to simply not use the collar and leash anymore, Severus wouldn't be able to do anything. Remus has power over Severus' pleasure through all those enchantments on the toys. Severus has power over Remus' pleasure, too. But it's freely given to him by Remus. (I hope I made sense. My English isn't the best.)
"I often wonder if I'm showing the D/s relationship correctly." I think so, yes. I'm by no means an expert, but I've read a few hundred D/s stories here and there. Every relationship has it's own rules and is different from any other. You don't have exactly the same relationship with your partner as your parents have/had with each other. (It's not possible, you are different persons.) Therefore not every D/s relationship has to be written in the same pattern. Keep writing, everything is fine.