What's the point with that quite lackluster story in Endwalker?

[Absimiliard]

You didn't answer the question. How can Venat gamble or lack control of something when she's well informed about the future of Emet and her imminent sundering slash?

Yoshi-P already answered that question in the link the poster provided. In short, the sundering required utilizing her power to its utmost limits. The monumental effort required made the technique something unwieldy to say the least - virtually uncontrollable, and so she was unsure if her attempt to give Emet-Selch an out would actually work. It was also partially up to the man himself; he needed to recognize what was happening, spot the "flaw" in the sundering, and take action to preserve himself.