nope, diend didn't have main riders card initially, he is copied all the kamenride,FFR and FAR cards that decade have while possesing the ride booker in eps 11, that's why kaito return it to tsukasa and tell him that he doesn't need it anymore. that should clear all this confusion.
I think that we are thinking too much into this...lol
yeah, you're all thinking too much about the reason why diend posses some of main heisei riders cards, and one reason that clear was he didn't take it from decade. So the only reason to explain why he can summon kiva and blade is, he has copied some of the cards within decade's ride booker. so i assume that diend have copied the main riders card that decade has already regain prior to faiz world.