Sessions can’t prosecute. He recused himself because he’s a potential witness. Any defendants would put him on the witness list to keep him recused. Basically, Sessions can’t do anything remotely related to Trump, the campaign or Hillary.
His only role is enforcing immigration and cracking down on pot. I’m somewhat surprised he hasn’t been stopped on immigration by the Dems claiming that his crackdown on illegals amounts to intimidation of Clinton voters in the past campaign. A liberal judge could make that leap of logic.
I don’t think Session’s recusal prevents him from ordering Rosenstein to deliver to Congressional oversight all documents they are requesting.