That would derail the investigation.

It’s telling that Trump’s team is just now putting it together that Mueller had their emails all along.

Rosenstein did write the memo giving false reasons for Comey's firing, and apparently realized at the time he was just being used for cover.
I think maybe he's learned his lesson, but he's certainly no pillar of integrity.

memos on both sides of the argument, and then DT used the one he preferred.

Another possibility is that when DT asked him to write the memo, Rosenstein consulted with Comey and they made a plan.

But his appointing Mueller so quickly (almost immediately -- Mueller wrapped things up at the law firm a week before he was officially hired) and his continuing to defend him convinces me that Rosenstein's on the up and up.

Link?