I'd move Keenum while he still has value. Start the kid, and Mannion backs up. Case is a free agent next year so I'd dangle him to the Jets or a similar situation and take something mediocre in exchange for him.
My reasoning: this team is not going to do anything significant, other than letting everyone down. I have seen enough, personally, in 4.5 years of the Jeff Fisher show. I'm officially done hoping he's going to be better than his record. I'd fire Fisher, trade Case, and let Williams be the interim the rest of the season with the rest of the staff in place. Hire a guy like Shanahan in the offseason who can give us a run game and handle a young QB correctly, maybe even hire him to replace Fisher immediately, hell, it's not like Mike's doing anything.
But that won't happen. Fish will get the remainder of this year, the team will slug its way to another sub .500 record while he coddles them and makes excuses for their play, and he'll probably even get an extension since he will blame it on the injuries.