you turn R instead of L and fly into a mountain
Very good point. Terrain is part of the briefing with and without engine failure together with cleanup, emg. turn etc.
Depressurisation? Get on oxygen take a couple of seconds to assess the situation - don't descend onto traffic, terrain - the escape route is open next to you - isn't it?
The next bits are secondary.
When does the FAA want the depressurisation briefing to be carried out?
Waste of valuable time before departure - how about during the climb?