The ATPL theory credit will only remain valid if you obtain an IR(A) within 36 months of passing your last exam AND it will then remain live for up to 7 years after passing your IR.
Otherwise your assessment is about right; I recommend that you get hold of a copy of LASORS (£10 - see the CAA website) as it will give you the full details.
But make sure that you'll be able to pass a Class One medical before you go too far down the commercial route!