Who's just talking about pistons?
Everyone else. Your point about turbines is well made, but I never saw a mixture control in the cockpit of a turbine aircraft, and that's been the guts of much of the discussion. Sorry, I have no idea about FAR23, or what it might mean to this thread.
Technically with an engine shutdown and feathered you are in an emergency situation.
Are you saying that as personal opinion or regulatory requirement ? In either case I don't agree with you, and nor does CAO 20.6 or anything else in the Regs that I know of.