could a small group of bandits be set to 'lure' a lord away from the lair?

Not well really. It comes back to the limited control over AI parties on the world map issue.

how about a trigger of a small group that is behind lord to try and lure them away. may not work but it would be an attempt anyways.

I think we'd run into a ping-pong effect on the lord's AI unless something else made him change his plans. If he's traveling from A -> B and he'll be crossing next to a lair then these bandits would spawn and he -might- chase them, but as soon as they are either removed or defeated then he'll resume heading towards B causing him to come near the lair again. So now the improvement kicks out another little group to lure him away and the cycle repeats. This could cause lords to get hung up on the map instead of furthering their intended objectives so it'd tip the balance of difficulty in the player's favor.

very true, unless there was a limit on how many decoys could be sent out. each one could take the lowest ranks from the existing garrison. or garrison could ambush the lords party depending on size. similiar to castles / towns sending out troops to help friendlies.