To clarify on the belt thing: for something as simple as a transformation device, it does way too much stuff. I mean, even in a KR series, the belt is not the one that does all the stuff. It's usually the Rider and/or their weapon.
Also, the Rider part I wouldn't mind, if he didn't take it too seriously. I mean, there *are* Kamen Rider parodies and homages in media, but here, it's explicitly Kamen Rider.
Also, Riders *do* trade stats for others with their different formes. So, if he goes to defense, his attack and a lot of his speed should go down. And for speed, his attacks gets lower, but his defense is basically non-existent. It's basic power balance.
Also, wasn't Ruby's purpose to simply give Rin access to mana and skills from alternate versions of herself or something like that? The Magical Girl schtick simply being Ruby trolling? Even if we go with Rin reverse-engineering the stick, that part wouldn't be the one she replicated