Plenty of people pass IR the first time and some guys find it easier than the CPL. The reason was is that IR is black and white 'do this and do that' whilst the CPL is more of an advanced PPL and is more colourful.
You don't do an IMC btw. PPL > Night > ATPL > Build hours > CPL > ME > IR
Some do the ME before CPL.
That's just the way it's done, typically. PPL is your basics, then you need a night rating but you can do this before your CPL if you want.
PPL is a requirement for ATPLs so that has to be done first, and then it's best to get all the theory out of the way and then concentrate on flying. Some study and hour build at the same time.
You really can do it in any order but to the do the CPL you need to do CPL exams, (12 exams?? somebody correct me) these are similar to the ATPL exams. And after all you want an ATPL one day so you may as well do all the ATPL exams at once to save time and effort. So that's why we do ATPL before CPL/IR.
People do the IR last because the CPL follows on from the PPL. IR and PPL are very different, and as I said CPL is like an advanced PPL. It all follows on nicely.
Anyone that says you won't get a job modular is a big fat liar.