The Leveon Bell situation has nothing to do with Tomlin and everything to do with money. Bell's not perfect, but that's on the GM and Bell's side to get that deal done or move on from him last year. It's not Tomlin's fault they couldn't get it done.
As for Brown, Tomlin can get his part of the blame for enabling Brown to this extent but the overwhelming reason this has happened is that Antonio Brown is an extremely immature person who just happens to be one of the best WRs of this generation. The reality is most WRs don't spend their career on one team because many have similar attributes including that "diva" attitude. Guys like Owens & Moss moved around during their career even though they were two of the best ever. We have been extremely lucky in the NFL to have a string of very classy elite WRs like Calvin, Fitz, and even someone like Andre Johnson. Brown isn't that kind of guy.
There's much more to this, though. I really think Antonio Brown is devolving as a person and these issues are going to keep following him. I don't know what's going to happen next, but there will be a "next" with Brown. He is a mess right now and I have real questions how he handles himself later in his career when he might not be the top guy on a team.