1. There is plenty of racial tension in DnD. Especially if the DM plays the world well. Take a tiefling or a yuan-ti to a tavern and see what happens. Or a warforged in Eberron.
2. Not everyone uses the same currency - there are some currencies that are only valid inside certain cities etc. But it's like how most people speak a dialect of Common - it's just to make life and trade easier.
3. NO. Magic-users never forget a spell. Cantrips are easy to cast, so they are unlimited, but the more complicated spells are tiring and take a lot of effort to cast. You are literally rewriting the fabric of reality. Magic-users need to rest to recover from complicated spells. It literally says that in the PHB.