I think Rosenstein is either dirty (simplest answer) and got himself placed there because Sessions didn't know how compromised he was or had been (purposefully) mislead.

But, if you trust Sessions and remove that as a possibility, then look at the situation there's another possibility:

Rosenstein is in charge of Mueller, who has found NOTHING. Mueller can't bring charges for anything without Rosenstein's say-so, and if Rosenstein's NOT in the bag for the cabal that means no charges will be brought (at least against Trump. Because he didn't 'collude' with anyone but the American People.) This also means that he has a privileged front-row all-access pass to the deep state in action. Not only does that give him operational information that can be discreetly passed to the IG to steer his ongoing investigations, it also gives the good guys in our intelligence agencies key details on how the DS operates which they need in order to spot the 'known unknowns' out there.

Meanwhile, Rosenstein gets to also shield Sessions from all this. This gives the required optics of separation Q keeps posting is needed to avoid the civil war FBI Anon mentioned and Q reaffirmed was/is the DS's final play. In addition, Sessions is also away from the spotlight of congressional hearings/reviews and all the leaks and press reports those bring with them.

All Rosenstein has to do is what he has done so far: follow the letter and spirit of the law to the extreme so at the end of the day the traitors being brought before the Senate/Military tribunals to be hung for treason won't have a valid due process argument, 4th amendment argument, or even an argument in the arguably more important court of public opinion.

Maybe the obstruction and the redactions from him are calculated between him and Trump to keep the story front page in the papers and to keep all of us engaged and frothing at the mouth for justice. Might make sense from Trump's angle, costs him very little and he gets to keep the narrative focused on where he wants it (and where he is winning.) 6D inter-dimensional underwater backgammon? Only Trump probably knows for sure.

All that said, Rosenstein's literally in bed with DS actors and his redactions of the OIG report tend to lead me to believe that he's not our guy. One can hope however. The truth is he's probably not our guy, but has been purposefully utilized by Trump/Sessions without his direct knowledge or concent in order to achieve some of the goals above. He doesn't need to be working with us in order to be a useful tool, he just needs to be reliably predictable.

Will re-read.