The monsters inside monster camps will stay there and protect their lair. It's quite rare that they attack a nearby unit. They spawn new monsters that will roam the map which in turn can attack your units and cities. Stardock has toned down city attacks by roaming monsters because they used to be too devastating due to the fact that cities and their underlying tile yields are lost once they have been razed by a monster.
The mechanic is working like this now: Monsters roam the map arbitrarily and they don't actively pursue a city, but accidentally "attack" your cities because they moved to a tile that is occupied by one of your cities. It's a bit different for units outside of your cities though, monsters will actively pursue them depending on the difficulty level. In the end monster attacks have more to do with chance now. You might be lucky moving and settling along monster camps or you might be unlucky, the world difficulty level is the lever that increases or decreases that chance.
AI's declearing war and not attacking is a semi bug. At the moment AI behaviour is pretty straight forward and they mostly look at your power rating to assess their relationship towards you and whether they should declare war, even if they are far away from you or don't have the means to actually hurt you. If they perceive you as a lesser faction, they get war hungry most of the time. Also, they might have been simply bought by another faction to declare war on you. The 1.5 patch is supposed to improve the diplomacy side of the game and hopefully AI behaviour too.
