Good tips, fbe. It takes a while to figure out all the factors affecting population happiness Nightwindking, and occasionally the displays are hard to read even for a veteran, but there is a lot of info there. If you didn't know about the spread of religion, now you know to keep a much better eye on it. Christianity is a really volatile thing in this game, and mastering it (whether playing Buddhist or Christian) is a lot of fun.

I'm sure there are many guides to agents to be found if you search. Some tips are:
  1. Monks and Priests are fairly easily level-able by siccing them on armies (demoralize). The smaller the stack, the easier to demoralize and less chance of failure (which often equals death for a beginner priest).
  2. Likewise, ninja sabotaging armies is an easy choice. Again, smaller armies are better (cheaper and more chance of success). Sabotaging buildings is also a choice; the lower the level of the building (less improved), the less the cost and higher the chance of success. Just click on all the choices you have in range and choose the easiest and/or cheapest, when training your ninja.
  3. I find metsuke the hardest to train because they mainly need to confront other agents... and unlike armies and buildings, enemy agents can be hard to predict. Often enough, but hardly always, AIs generate them at their capitols.
For the record, your agents get 15 points for a successful action and 3 for a failed one. Also, 1 point/turn for a "maintenance" action (like a metsuke being embedded in a town to increase its taxes).

Also for the record, I don't believe the percentages shown in game are always right, particularly at the lower ends (below 50% chance). Above 80%, they seem to be on target. Also doing something like trying to assassinate a daimyo seems to be really overstated; you probably only have a third of the chance shown.

Something that may not be obvious is that any failed action against an AI also incurs a diplomacy hit, which you can see if you mouse over the other AI in the diplomacy screen to see the reasons for your current score against them. If you want to worry about this, you can choose to harrass rebels and/or only war enemies... but the point where AIs decide to war with you seems to be complex, so I'm not yet sure how much it matters. For one thing, they seem to declare war first if someone is adjacent to them, and only secondly based on warscore if they have no one to declare war on, next to them.

Sometimes early in the game I will have a ton of young monks and ninjas around one hapless one-unit stack, which can't move and is incredibly demoralized. Poor bastards, ha ha.