I believe I have puzzled out the purpose of Ituralde's strike at the Seanchan. I need to credit ajfurst for suggesting the strike at Durlana is not going to effectively be large enough to take out the whole 150K Seanchan troops and therefore probably a suicide mission.

Just a short recap of the evidence from the prologue.
1.) Ituralde notes that this Seanchan army is sandwiched between two large forces of equal or greater manpower.
2.) Ituralde thinks back on the bitter lesson of warfare that following orders doesn't mean living, and often doesn't even mean winning.
3.) Ituralde and all the men with him are not wearing thier uniforms because they are needed elsewhere.

So if you are Ituralde and have slapped the Seanchan bull on the nose. You are now running back to the ground you chose to turn and fight on. (warefare 101) But you are suprised by one Dragon Reborn and an extremely large army apearing out of thin air in front of you, so you need to change your plan. The Seanchan army is large and with the 100 suldam/damane, they have you licked in a fair fight, so what do you do? If I were Ituralde I would use the other army to my advantage.

The forces hidden in the relatively medium sized city of Durlana are not going to be anywhere close to 150K, but you could man the walls with say 10k hidden men and put up a good fight for some time if the Seanchan are commited to getting into the city, which they are since they are caught between two very large armies, and Durlana is the only true "refuge" for them.

Ituralde always lays plans within plans, and this ambush is not the least of them. If you could convince the Seanchan that they had defeated one of the great battle commanders leading one army, wouldn't you expect them to focus thier attention on the other one.

Ituralde is setting up the story that the Seanchan have come across his personal secret holdout for this area and is making a last stand in the form of another ambush, much like his entire battle plan for this campaign so far. (with ambushes the entire length of Almoth Plain and into Tarabon directed at the Seanchan forces) He gives his uniforms to the commanding soldiers within Durlana so that when the suicide mission is over, the Seanchan feel like they have lopped off the head of the snake.

Ituralde also is going to hurt thier one power force with those "farmers with bows", but that is just a bonus and a plan that may or may not succeed based on the reaction time of the suldam/damane to this ambush. But no matter what Ituralde is going to gain some time when the Seanchan believe that they have killed him and even momentarily turn thier back on him as they look to face the other large army.

The Dude
(bowling is this age's stones game)