Having done this many times, I can not imagine how you can properly
prepare a candidate for instructing( notice I did not say passing the
practical test) in the time you describe.

I can't. It can't be done in a weekend. Or a week. Or a month.
Takes much longer - takes consistent development and mentoring over the
course of years. And that's my point - you're either turning out a
credible instructor, or you're just preparing somoene to take the test.
If your involvement is measured in anything less than years, it's the
latter. So why worry about quality? It's either there to begin with
(meaning the candidate is already a credible teacher when you get him)
or it's not. He'll pass the test either way.

The difference, in my view, is the experience needed to evaluate the
candidate and tailor the training program to the candidate's situation.

Evaluating a candidate's teaching ability is easy. Primary students
can do that effectively - they just can't evaluate the subject matter
expertise (since they don't know the subject matter). Some people are
good teachers, some are not. You can't take someone who is not and
make him into someone who is in a a few weeks time.

I know people who have gone off to two week CFI schools taught by
people who churn out many CFI's - both glider and power. The result is
always the same. They all come back with a ticket. Those who could
teach before still can, and those who couldn't still can't.

I've also known some people who learned to instruct - but that was
always in the local area, by long term mentoring.

Michael