On the subject of accidentally pulling the mixture to ICO, it's worth thinking about this on the ground and deciding what you would do to recover the situation.
In theory, it should be a non-event. Even if you've pulled it back all the way and really "killed" the engine, your prop will still be turning, and therefore the magnetos are still sparking. You're still getting air into the engine, so the only thing that's missing is the fuel. If you restore the fuel mixture, the engine absolutely MUST burst back into life - in theory at least
So just gently and smoothly pushing the mixture control all the way back to fully rich without touching any other control should do the trick.
I guess there may be engines where this might not work (I can't quite decide about injected engines, for example) - but I bet it would work in all bog-standard trainers.