I think you are using CRM out of context.
What I think you are looking for is a general set of SOP's for two pilots to fly a single pilot aircraft.
The brief to the RHS pax in any dual control aircraft be it qualified or unqualified, commercial or private ops should include a "don't fecking touch anything without me asking you to" part.
Myself I wouldn't dream of touching anything unless the **** was hitting the fan and I could do something about it.(which is very unlikely because I would have highlighted way before there was a need) and then it would be "I have control".
pretty horrible situation to be in PACE I think afterwards I would be in such a state of shock I wouldn't have know what to say to them.