Would I need chintugs to stop a QR wheel slipping?

Is the mech hangar replaceable? (it doesnt look it)

Yes I think you do need chaintugs. Although I run my singlespeed commuter without them and use deore QR done up nice and tight. Probably more forces off road.

The mech hanger is not replacable. I have the 456 not the slot dropout. I bent the hanger but fixed it with a large adjustable spanner to bend it back. It's thick chromoly so it's unlikely to break. Had my 456 for nearly 3 years now without problems.