Assuming you have a PPL, if you got the IR, then the exams would be valid for 7 years, so doing it backwards is an option.
This is incorrect.
Once you get the exams, they remain valid for 36 months for CPL & IR issue.
Assuming you get both, they remain valid for 7 years from the last IR check for ATPL issue. They will not be valid for CPL issue after 3 years.
EASA FCL.025 spells this out very clearly:
FCL.025 Theoretical knowledge examinations for the issue of licences and ratings
(c) Validity period
(1) The successful completion of the theoretical knowledge examinations will be valid:
(i) for the issue of a light aircraft pilot licence, a private pilot licence, a sailplane pilot
licence or a balloon pilot licence, for a period of 24 months;
(ii) for the issue of a commercial pilot licence, instrument rating (IR) or en route
instrument rating (EIR), for a period of 36 months;
(iii) the periods in (i) and (ii) shall be counted from the day when the pilot
successfully completes the theoretical knowledge examination, in accordance with
(b)(2).
(2) The completion of the airline transport pilot licence (ATPL) theoretical knowledge
examinations will remain valid for the issue of an ATPL for a period of 7 years from the last
validity date of:
(i) an IR entered in the licence; or
(ii) in the case of helicopters, a helicopter’s type rating entered in that licence.
Therefore if you do not get BOTH and CPL and an IR within the 36 months, you would have to retake ALL the exams.
https://www.easa.europa.eu/system/fi...u/Part-FCL.pdf