Trade can be a royal pain in the ass to get in some campaigns. One thing to note is how your status is to other factions. If you are classed as untrustworthy they wont trade so work on maintaining a reliable trait (this is shown in green, yellow or red writing above your leaders avatar on the bottom left of your screen). You need to also look carefully at what the faction likes/dislikes. If you can then see that a faction has allies or client states try to work diplomacy with them as well as it will improve all the factions status with you. Key thing with trade though is as Gussie above mentioned, trade resources. If you have no resources to trade you wont get many trade agreements, so focus on taking settlements that have trade resources and try again. there is always the option to offer a faction some money for trade as well, but only do this if the faction is reliable. It may cost you a few 100 talents but you earn that back easily from the trade.
As for your question regarding war. If its a guaranteed win and the losses will be low just auto resolve. But if the balance of power (especially in defensive fights) is in favor of the enemy then fight the battle out. You will be surprised how well you can do sometimes, and even if you lose you will likely do more damage to the enemy force than an auto resolve will do, making taking the settlement back an easier task for near by forces. Always remember though that an auto resolve can sometimes kill off more troops than you would lose by fighting the battle yourself. Its not too much of an issue but it could be the difference between 1 turn of recovery against 10 turns. In a fast pace campaign where losing as little troops as possible is important you really need to be doing the battles.
Fell free to add me to steam or send me a message if you need help with anything else with your game play

You could also check out my youtube page for some of the lets plays that I do (shameless pun I know

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