UA - The second part of your question wasn't answered:
Yes, you can just do the CPL within the three years. Or you could just do your IR. Or you could do both. But you won't be able to get either once your exams have expired.
You don't need to have completed your ATPL to keep the constituant parts valid.
You do not even need an IR or CPL to instruct... you just need to have completed the CPL/ATPL exams and passed the FI course. You just can't earn money by instructing without the CPL.