Hmm, I must have both a different game and a different version of EB.. Because the AI does not behave dumbly at all, and certainly does not backstab.
I have had an alliance with Epeiros, 6th most powerful faction, my Getics being the third, for 70 years in my campaign - we share a border of 4 provinces, three of those on my part being defended by a garrison of one company.
They have never hinted at betrayal, and often stand before my cities and defend them from the now barbarous Makhedonians.

I think you simply have to treat the AI with respect, odd as that may feel, and see the bright side of it's suspicion, if I were playing a game where everybody's object is to eventually destroy or reduce everyone, friend or foe, I would be cautious also - and I am.
I do my best to think of the AI as a human player, use the diplomacy system to figure as complicated deals as I can, and the AI often does some extraordinary things in reply.