I tend to think that that night was a whirlwind of emotions for Michael.
I think Michael already was suspicious of Fredo, perhaps not consciously, when they got to Havana. The uncharacteristic show of emotion with the kiss of death is because it really did break his heart. It's lile being punched in the stomach: "My own brother....betrayed me." But then when everything was in an uproar and danger was all about, the tiny little bit of Michael's humanity peeked through - when he said, "You're still my brother!", he meant it. Just as the kiss of death was a very heat of the moment, emotional thing, so was him wanting to make sure Fredo was safe.

I don't think Michael had fully digested the betrayal mentally that night, and I think he was in a massive state of emotional turmoil. So I don't think he had any intent to pump Fredo for info then and there.

Fredo hiding away in NY, which made him look weak and guilty, and making it worse by lying the level of his about his involvement with Roth, then revealing his deep seated resentment of Mike and his sentiment that HE should've been named Boss, on top of not telling Michael that the Senate lawyer was in Roth's pocket (which says to Michael that Fredo betrayed him TWICE - by not coming forward with the fact that the Senate laywer was in Roth's pocket, he's helping Roth set Michael up AGAIN).

This period separation helped end any fraternal love he felt. It hardened his heart, and made him view Fredo not as a feebled minded, misled brother that messed up (as he might've still viewed in Havana) but as a conniving weak man and very real threat. He may have also in some subconscious way blamed Fredo for the downfall of his marriage with Kay, which one could argue really began its deep slide after the assassination attempt.