IMO his sudden desire for cat food is caused by his finally finding it. I think captive iguanas seldom get a good combination of vegetable food that matches the nutrition they find in the wild. So, he might be hungry, and takes it out on kibble. It looks like you are managing his food dilemma well. Mainly, no brassicas.
Agitation and aggression are common when you put an ig in a new place. It will slowly wear off.
Hope this helps,