Can you fly a US 'homebuilt/kit a/c' on N-reg with an FAA license in Europe ?
I take it you mean the Experimental category. Yes but being sub-ICAO you need a permission from every airspace owner.
And if yes, can you fly IFR (obviously holding the required FAA license & rating). Truth be told, I'm not even sure if kitplanes are legal for IFR in the US, but....
They are OK for IFR - it depends on whether the inspector signed it as being ok for IFR. Whether they are
ever OK for IFR over here (even with the permission) I don't know (I doubt it).