I have followed enough cases to know that the 1st plea is typically always "not guilty." It just gives his new attorney time to read all the evidence they have against him. And there is a lot. Not even counting what hasn't even been released yet.
With CWW hand feeding LE all of the missing pieces of the puzzle, Marks going down regardless of how he pleads. Even his own friend of 30 years (CK) who knew NONE of the evidence LE already had was convinced he had something to do with his wife's murder. I'm sure Mark never saw that one coming. CWW will be able to fill in all of the information LE needs, for example what was said on the burner phones, the 100 dollar bills used in Florida from the "gift" that Mark sent and wanted to make sure it didn't fall out of the card. We know for a fact that CWW didn't even have enough money to buy groceries, and all of the sudden he and JR both have 100 bills they used paying for items that were CAPTURED on surveillance. I personally don't think he will take this to trial simply because of CWW rolling on him. However I don't think he will EVER admit to this murder, but plead no contest instead. But then again he is a narcissistic psychopath, so he may think he can pull it off. JMO