1- Even if it wasn't Marcus' call, he still went along with it and then he stupidly called out Susie on her intention to vote out Corinne instead of reassuring her.

2- It's tough to be good at Math when you don't have the problem straight. By "calling their bluff" as he said, Marcus made a majority of players angry at him. Matty, Sugar, Kenny and Crystal expressed their displeasure and who knows how Susie and Bob would have felt about the idol.

Not only do you have the problem wrong but you seem to have forgotten the lessons of Fiji, China and Micronesia. Someone other than Marcus could have found the idol but so what? They would have become a bigger target than Marcus. An idol is so easy to flush that Marcus had no reason to impose his will on others. That's Survivor Suicide.

In the end, Randy having the idol in his hands was in no way bad for Marcus at this stage.
(and yes, Marcus did have the idol in hand because he was the one that threw it out to sea!)

3- Mingling with the opposition during her birthday party was crucial to Danni's win and no one in her tribe thought she was doing it to flip.

4- Marcus didn't have to say anything to Crystal about voting out Kenny. He only needed Bob and Susie. His surprise connection to Crystal could have made him go with voting out Susie. Instead of chosing one or the other, he tried to have it both ways. Dumb.

5- Don't you see the difference between Marcus and Rob's situations? Rob wasn't in immediate danger, he didn't need to make a Final 2 deal with Alicia. Marcus was in mortal danger the moment he was switched. Look at the scene where Susie asks him for a top 3 deal. Marcus says nothing and then Susie has a confessional: “If I do switch, it’s his own fault because he didn’t give me a clear statement.”

6- "If your original tribe has the numeric advantge, why not talk about keeping that advantage strong?"

Marcus told Jeff: I think it’s important to keep the tribe physically strong and ready for challenges” with Susie sitting right beside him. Don't you think she felt that as a threat? He didn't say he wanted to keep his numbers. In fact he denied it when Kenny accused him of BS.
Bob didn't help when he said he made a mistake voting out Dan when Susie was the alternative.