That leads me onto another question; when judging whether an aircraft should be multi-crew or not by its weight, what is used: empty weight or MAUW
Not sure it is that clear cut. I stand to be corrected but think that the airlines OPS manual and/or the AOC dictate whether a specific aircraft type has to be flown multi crew. I seem to remember in the distant past reading somewhere that the CAA have a list of aircraft types that are NOT considered to be multi crew. I would think this is dictated in turn by a mix of the aircraft type and the operation/purpose for which it is flown.
The whole hour building on a twin turbine is a tough one to really answer in a lot of respects. Some operators might give you "credit" when reviewing your cv irrespective of whether or not you could log the time for JAA. Of course you would have to find a way of drawing their attention to these 500 hours not included in your JAA totals. Others might not be so kind. I guess a small biz operator from a regional airport MIGHT be interested if you could prove to him that you have 500 hours in the RHS then again would you be JAA type rated? Type ratings do not necessarily cross transfer from FAA world to JAA land. Then there is the issue of whether your 500 hours are really what the operator is after and whether he is really after someone with European flying experience and more hours. Lots of ifs and buts that really don't have an answer I am afraid.