Hi Moheet,
1) You can do the FAA IR directly on the Multi *but* it will be cheaper to do it on the Single and then add a Multi to your FAA PPL. Once you have the Single IR the Multi checkride will include an instrument approach to add IR privilages on a Multi to your IR.
2) To convert a Multi FAA IR to a JAA Multi IR you will need to go through a 15 hour conversion course at an approved FTO plus JAA IR Skills Test. BUT more importantly before you undergo the training for the JAA IR you *MUST* have the passes in all of the JAA ATPL or JAA IR exams in your pocket.
3) If money was no object then I would still prefer to get the FAA IR Single and Multi first and then convert to the JAA IR. But then again I'm looking to be dual qualified across the FAA and JAA licences / certificates.
4) Do you mean JAA ATPL exams? If you are looking to do the JAA ATPL exam then you might as well do those instead of the JAA IR exams. You *have* to have either set of exams before the JAA IR course.
Once you have the JAA ATPL Exams Passed and the JAA IR you will have a JAA PPL/Multi/IR. To convert this to a CPL/IR on Single and Multi you will have to go through a JAA CPL flight course and skills test. Again the JAA ATPL exams must be passed before you can do this.
You will then be issued with a JAA CPL/IR Single Multi with ATPL Exams Passed. This is otherwise known as a "Frozen ATPL" in JAA land or in US speak a Multi Commercial Instrument Rating.
Hope this helps.
Charlie Zulu.