I have never heard of any authority "scaling" hours flown in the US by 50% unless they have seen people trying to pass off somebody else's hours as their own and have lost patience with US time builders?
Agreed. Normally the just discredit the whole lot as they can't prove otherwise.
As has been covered
multiple times before on this forum it is debatable whether you can log any of the time JB describes under JAA - as you cannot set out on a flight knowing that someone else will be logging PIC as well and then log JAA flight time yourself. It only works if the other student/pilot logged no PIC.
This then of course can invalidate CPL training as they wouldn't have the mandatory minimum hours for the start of course.