I ran into the same problems when I first started playing it, took ages to find the answers too
1. Can recruit some faction troops from the commanders in towns/castles, they are found in the town centre. Can you recruit them as a merc? Not sure about that but you can as a faction lord.
2. Pillage enemy villages, you can make way more money from burning a couple villages and selling the loot than a caravan.
3. Try asking lords in other factions the answer to what was written on the cannons.
Hope this helps a bit.