To answer your first question, everyone (except Asta of course) has basic magic but their grimoires are where the intermediate and advance spells are accessed. That's why a lot of the other side characters would go "He's this powerful even without opening his grimoire?" when it comes to Yuno. It is possible to still use magic without it but to perform more powerful spells, you need to open your grimoire. In Asta's case, it's a little different because he has no magic in the first place, so he needs to have his grimoire open to access his swords.