Are you really saying that you received no instruction on student fault analysis during your FI courses? I find that very hard to believe.
My example of cost sharing was based on shared flying, not flying as a passenger. The CFI cannot decide which non-CRI/FI PPL pilot can conduct these 28 day flights no matter how dictatorial he wishes to be.
Quite simple - any additional Club recency requirements must be conducted by a CRI at the very least. Not merely by some unqualified PPL holder who happens to be thought of in high esteem by the CFI.
For example, we have a 42 day recency rule and an annual Club check rule. But those outside 42 day recency or needing to fly an annual Club check may only operate the ac with a FI acting as Commander.
I've discussed with others and they are equally adamant that the use of unqualified PPL holders as 'check pilots' is wholly unacceptable except within the agreed procedures of a private owner group.