Also, I want to address an inconsistency I've presented.
Once upon a time, I mentioned that Motonari likes working with change. Which is true. He uses changes in the world around him to force a weakness into his enemy and then exploits said weakness. Everything he does in SB3 revolves around that concept, from forming not one, but two agreements with Yoshitsugu (and subsequently breaking them both without hesitation), destroying Shikoku, and really just getting in on the action, even all the way on the other side of the country at times. It's said many times that he really doesn't go out much, but there you have it.
But see, his own spirit is pretty steadfast. That's where the anomaly arises, because while he likes change and chaos, he avoids it on a personal level. He has these undying, unyielding beliefs that he will cling to, and he has his own little domain over in Chuugoku-- and unlike anyone else, he's really not interested in taking over anything else. (Which he makes sure to state often.) So really, no matter what happens to anyone else, he does his best to insure that his own life isn't going to undergo much drastic alteration. And I think that's really a lot of the reason he just said fuck it and helped take down Nobunaga in the anime, even though he had both Keiji and Motochika hassling him and he'd probably really rather just tear big gaping holes in their hides for messing with him.
And that's why there seems to be inconsistency here. My fault for not ever explaining.