Foz, if you choose to become a dedicated career instructor you will never fly multi crew, as such you will never obtain the 500 hours multi crew needed to unfreeze the ATPL; as such if you remain a CPL/ME IR instructor you won't need the ATPL credits.
So if the ME/IR instruction route is the career path you choose then the lack of ATPL credits will not be a problem. In other words with just a CPL/IR and FI you will quite happily make a career as a multi IR instructor.
If the worst happens and you can't get an extension (hope you do by the way) remember with a CPL/IR and no ATPL credits you can still fly Single Pilot Aeroplane types; hence you could still get employed on a Twin Otter, King Air or even some of the smaller citations. Infact some Air taxi firms and flight schools may see the lack of ATPL credits an advantage as it means you are not able to move onto the airlines.