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There is likely some sort of plot twist in store... something none of us are expecting.
Why is the blackmail theme so weak for this story?
Well what damage does such blackmail do if revealed? So Marvin gets footage of Anna with say Jake. What is the worst that it can do if that gets out? If Anna is getting railed by Jake (or even Hank), Anna likely already has one foot out of the door with her marriage. It will not cost her job unless because of the embarrassment. But she still has a law license. And we are forgetting other outside factors, like Grant. See two story plot lines can be connected.
The only way Grant can get back together with Anna is if he gives back her father's estate back to her family. Anna isn't going to let Grant get near her unless somehow, he makes right the past. So perhaps the money end is covered.
Now where blackmail would be effective is if there are serious consequences involved. Suppose Anna was caught embezzling or doing something really illegal. Let's say that happens. The other weakness of Marvin using blackmail is what hold does Kendra have over Marvin to give her that information... if anything everything Kendra hopes to accomplish is moot because Marvin will keep it all for himself. HOWEVER.... what if Marvin is able to record a conversation with Kendra, damning information, and uses those conversations on Kendra. I doubt Marvin wouldn't kick Kendra out of his bed. Well not up until she puts a bullet through his brains.
To answer the question why Marvin is spying on Anna is to perv off of it. It's for his own amusement. Now it's not always true the Enemy of my Enemy is my ally. Think 1941 Ukraine, Germany verse the Soviet Union. Pre-1940 Russia starved to death some 6 million Ukrainians... but turned out the Germans were worst. (sorry for the history lesson)