It sounds as if Flarm would have been an excellent system if it had been adopted internationally, no need for an expensive transponder to provide ADS-B out, and I think that it would be better received by pilots than the US mandate for is ADS-B on cost grounds alone. The cost of meeting the 2020 requirement in the US will exceed the value of some older aircraft that operate around Class B, C and D airspace.
ADS-B is intended to replace ground based radar in many areas, and will be an essential part of the Air Traffic Control system. Do European ATC centers have access to Flarm data, which could then be pushed out to ADS-B users and so prevent conflicts?