Once the interim system is out of the way I suspect you will have to do a bridging exam with a "composite paper" and a couple of other subjects (got approval for that too.)
In answer to some questions:
1) The lessons of the school offering ATPL(H) will be slightly different from the school offering ATPL. ( H - focus on helicopters )
Yes, POF and performance will be different, no questions on IRS operation (there are on the inner workings) and a couple of others, like pressurisation
2) No matter what school I chose I will take the same exams. (that are the same for airplanes and helicopters - but no questions for helicopters in the exams).
The exam questions for helicopter exams are helicopter specific in the UK
3) So if i chose the ATPL(H) I will propably have more to read (airplane+helicopter) but I will improve my helicopter knowledge
There is no studying for fixed wing, except where it illustrates a helicopter point. Certainly no 737s! Super Puma instead!
Phil