... at which point the CAA is likely to investigate the circumstances of the appeal.
I agree that if the candidate has met the requirements, a logbook/licence should be signed. If not, it should be justifiable why not, in which case the instructor has nothing to fear from the CAA. In my book, "dangerously weak and won't listen to advice" could potentially be grounds for that justification.
Equally, I don't believe a revalidation flight should take place without it being made clear to the candidate that there may be circumstances where a signature will not result. Foxmoth makes a particularly valid point in this regard with reference to a new experience for the candidate, where more discretion can probably be exercised by the instructor, versus a familiar situation.