With regards to the slapping, I agree that there is a he-said-he-said back and forth going on. And really, in the midst of the cult insanity, Marty did a fair amount of slapping as well.
I find it hard to believe that Marty couldn't lead them to hard evidence if he really wanted, especially with regards to some of the financial misdeeds. He's not stupid. Same with Mike - the guy was a high ranking OSA official, for fuck's sake. I think that DM does a good job covering his link to OSA shenanigans, but even the best criminal minds slip up occasionally.
I also think he knows about literal bodies being dumped. He helped with the Lisa cover up, and there have been oodles of deaths at Flag. Also oodles of mysterious deaths around the Co$ - Flo Barnett anyone?