I need to get the Flight Instructor Certificate -> which requires the CPL
No it doesn't (assuming that you mean a JAA FI
rating). Neither the night qualification nor the CPL(A) is a pre-requisite for the issue of an FI(A) rating. You will need to pass the theoretical knowledge examinations at CPL level but the licence itself is only necessary if you expect to be remunerated for flight instruction. Since the restrictions on your medical mean that you will not be able to instruct much beyond the basic PPL level, I assume that you are not expecting to make a living from flight instruction.