Ultimately the person I am 'checking' is PIC, as they have an SEP rating attached to their licence which allows them to act as such. I am simply a passenger, offering advice when required. And should all else fail, a briefing beforehand allows me to regain control - "I have control."
Let's start with the absolute no doubt part of the above. As a passenger you can never say "I have control" (unless you want to be accused of hijack). You are a passenger and have no right beyond that of a passenger who is in an aircraft for the first time.
As soon as you let anyone be PIC of your aircraft in a particular situation then you are saying that you are happy that they are qualified to be in that situation and no further training is required. Having established this then there should be no difference between you being in the aircraft, a non-flier being in the aircraft or even the PIC flying solo in the case you describe.
So if there is an accident - say an over-run into the far hedge on one of these short strips in the example you gave.
Q1. Was the pilot qualified to fly into that strip? (Not just licensing terms but recency, and club/syndicate rules also)
Q2. If the pilot was not qualified then why did you let them take your aircraft and a passenger there?
Q3. If they were qualified then why did you feel that you needed to be there to be "offering advice when required. And should all else fail, a briefing beforehand allows me to regain control - "I have control." This seems to be a contradictary position......and I am sure your insurance company would love to see that!!
In simple terms you can't have it both ways.
Now if you were to be pilot in command and you let your passenger (who holds a valid licence) fly the aircraft into these strips you would have the ability to say "I have control" you would have the ability to "offer advice" you would have the ability to "tell them what to do"......and if you run off the end the PIC was fully insured and current at flying into such short strips.
Of course you would have to pay atleast 50% of the flight costs. Is that a problem? The passenger could also pay 50% but they don't log anything other than the take-off and landings they did as sole manipulator. Is that a problem?
In simple terms - if someone needs training - get an instructor. If they don't then let them fly. But please don't let people who you have decided need instruction fly without training becasue you are leaving yourself open to all sorts of issues.