My opinion is that the Examiner can make sufficient assessment of an experienced FAA IR holder by observing the applicant's pre-flight planning as well as the practical in-flight conduct of the C-B IR Skill Test, perhaps with a few oral questions where necessary. Which, incidentally, is the way I used to assess pilots when I was conducting IR tests on the VC10 in the RAF.
Surely for a 3rd country IR, the fact that the applicant has already passed both theoretical and oral as well as flight test for the 3rd country IR should be considered sufficient "theoretical knowledge".....as it is now for the IMC rating. (EDIT: I have just re-read what you said and that is exactly what you said
)...
The oral and flight test should be sufficient for the examiner to accurately gauge the person.