Hey
The way it goes is you need ( for Europe ) a JAR CPL( needs to be done in a JAA approved school) a multi-engine rating is just that, a rating so you can do that in the US and as far as I know does not have to be done under the JAA system, and a SINGLE engine IR is not enough IF you wish to continue to ATPL level. again you will need a multi -engine instrument rating which you will complete after doing your multi-engine rating and for European license this must be done in Europe, because to make your life even more difficult they assume to have different standards. So you will have to pass your multi engine instrument flight test in Europe ( at least that is the way it was for me a few years ago). Your CPL you must do in a JAR school but can be done stateside. I am sure that is clear as mud, maybe someone who is currently doing it can explain better.
Good luck
PS your commercial tarining and test may be done on single engine but the aircraft must have variable pitch prop and retractable gear!