So don't worry about the ATPL exams. For the purpose of issuing an ATPL you only need to maintain a type rating on any helicopter to keep them valid (this is different to fixed wing). The IR part is as you have already said - you need to do your IR within 3 years and then you have 7 years after holding a valid IR to redo some of the exams (just the IR ones). So don't sweat it. Do your ME/IR then get a job (co-pilots HEMS perhaps - although very bad for hours building).
As for having an ATPL making a difference for jobs - the answer is it depends. For any single pilot onshore job it makes no difference. For an onshore co-pilot job it makes no difference. For an onshore multipilot onshore job you need one. But you also need over a 1000 hrs so you won't be there for a while. You also likely need to pass a command course Basically don't worry about it at the moment.
For Police and HEMS you need hours (it should be you need experience but that's not actually how it is). You need to find a role to get you those hours (instructing, charter, offshore). You will naturally keep your IR and ATPL theory credit ticking over in most jobs. Note you don't need an IR for the police so actually you could just do instructing then go police when you have the hours - not a recommended route though.
So, focus on your ME/IR, consider what you want to do (and enjoy) then start talking to people who can help you get there .