1. Everyone can use a buckler regardless of class or proficiencies. Reasoning: It is a natural reaction for a person flinching from A perceived attack to raise their arm to block it. Even a toddler in real life can do this. So there is no excuse why A commoner or mage in D&D can’t. Granted someone using a two-handed weapon such as a longbow will not be able to get the AC bonus from the buckler. And a wizard casting a spell with a somatic component cannot get the AC bonus. But anyone can use a buckler.
2. If you have bonus spell slots, for which you do not know a spell for, you may fill these bonus spell slots with lower level spells.