I see an entirely different perspective. Cruz is between a rock and a hard place. He is a conservative and he is a man of his word. When the situation is such that his integrity is affected, he will step down to keep his honor and be able to look himself in the mirror. I believe the 'cartel' made things abundantly clear to him. Just my opinion of course.
I agree with your take. There were two things to consider here, one regarding Cruz the man and the other regarding the country's welfare; the personal affront to Cruz and his family, and the policy PROMISES the Trump camp made to Cruz in exchange for his support.
Regarding the latter, I do not know what more we could realistically expect from a prominent conservative figure at this point. Cruz apparently used what little leverage he had. Those six point he laid out ARE his principles. What is not within his control is whether or not Trump honors them.
The truth is Cruz could gain us nothing more by sitting the election out.
Regarding the first consideration, personally I do not believe I could overlook Trump's insults to my family, but thats an entirely personal matter.