I'd appreciate your advice on what the CAA's stance is?
The Examiner is responsible for ensuring that the candidate is qualified in all respects for licence or rating issue, so unless its done within the 6 months they should not conduct the test. If they do, then the CAA will probably throw it out.
From my understanding, ... many lapse MEP and revalidate every other year
In which case it would be a Renewal not a Revalidation and require a Course Completion Certificate from an ATO before taking the Prof Check