Rich,
For conversion of FAA IR to JAA IR the requirements are in LASORS. I doubt you'll see any financial benefit from doing the FAA IR first.
3 years to get your IR after completion of ATPL exams, can't remember max timescale for ATPL study, most people seem to do 12 to 18 months.
If you satisfy the FULL requirements for issue of a CPL when you start the IR course, the course can be reduced to 50 hours (if you've passed the CPL but still need to do a few hours, for example, you have to do the full 55 hour IR).
If you do a CPL abroad you will also have to convert that to JAA.
As regards pick n mix training - some will tell you it doesn't make the slightest difference where your training is done. However, food for thought - FlyBe, probably the biggest recruiter in the UK at the minute - state a requirement for "one stop" modular, i.e. CPL and IR done at the same school.
Hope that offers some help.
Cheers
Neil