"How much would you aim to control the media around you?"
Considering they don't have internet or television in this post apocalyptic scenario, my gladiator games would be the only source of entertainment. I would be the media, I wouldn't have to control it if my performance in the arena is superb.
"Even with a scout, don't you worry about strong communication paired with personal identity and freedom forming a risk? What if the one you send out to scout is the one devious enough to arrange a mutiny?"
Of course, but as history has shown there's no real way to stop it completely from happening. It's why i want them to see me as a mentor and why I take the time to train them personally. It also helps that this religion I'm running is very strong on order and honor. Taking apt preventative steps is really all you can do.
"What if they splinter off into their own variant of your philosophy, one with a Justicar using it to justify a mutiny, a civil war?"
This is where the real trouble arises. I wouldn't cause bloodshed, it would only weaken my image. The only viable option would be to challenge this justiciar in the arena fairly. It runs the risk of me dying but not more so than fighting a civil war and when I win it'll instantly relate the fallen justiciar to the criminals that also met their ends in the arena. Silencing any of their sympathizers while bolstering discipline in my ranks and increasing my religious influence greatly amongst the common people.