Summary of LASORS section D & J state :-
Once you have passed ALL the ATPL exams you have 36 months in which you must obtain either a CPL or IR to keep your ATPL exam passes valid.
Then providing you obtain an IR within this period (36 months) the ATPL exams are valid for a period of 7 years from the date of your last IR renewal.
By which time the system will have changed again!
I STRONGLY recommend you do the theory first (ATPL) & get it done. There is no point in going spending thousands on your IR if you can't pass the theory exams.
I would NOT mix CPL/IR flying and theory, you are putting yourself under more stress, even integrated courses don't seem to do this anymore.