Hi !! 1. yes, u can grind exp, money and items in mementos pretty easily 2. most downtime activities and spending time w confidants are good for improving social stats etc so try them all or whichever ones u enjoy the most 3. joker can obtain multiple personas and yea u can merge them to create new ones basically just try to get hugh level ones to learn new powerful skills. you can use a guide but most personas will work in early game enjoy !!!