I think you are creating an issue that does not exist. I don't understand why you think you do CPL exams then ATPL exams separately. Just do the ATPL exams in the first place then all of the pain is over.(the difference is minimal) You then have a frozen ATPL instead of a CPL and as the years tick by it unfreezes.
(BTW you will get bored with instructing- especially CPL/IR -you never touch the controls, lesson 3 rocks)