About leveling up Leadership: the best way is to creates armies, with your companion's parties, so you don't need to use influence to call them. And just play and move everywhere with your army. You will raise up leadership everytime you fight or just move on campaign map. Also, if you wait inside a town with your army, you will improve relationship with traders and artesans from the town. Don't forget to have good variety of food and win battles, so your soldiers morale is always high.
About leveling Charm: If you have a lot of money, just give gifts to nobles. It will improve your relations with them and your charm will level up. When you fight against nobles, release them after battle. Big charm boost.
What version are you playing? Mantaining your own kingdom is hard, but it's possible. When you need to pay a lot of money to a lord to join your kingdom, that mean's the lord don't really think joining your kingdom is a good idea (too weak) and they leave very soon. Small clan lords, like clan Tier 2-3 are more likely to stay with you. In my last kingdom, when most of the lord joined me, they left my kindgom in 10 minutes. Only 3 different clans were really loyal to my kingdom.