My thoughts for what they're worth...
(1) The system as introduced is safe insofar as it simply took the most stringent parts of each country's systems and combined them. This inevitably added cost and complexity at every level. Most of the individual systems worked adequately as a whole, and this was never properly recognised. Result - mild to severe chaos.
(2) Only in parts, I find it useful to be able to fly a German registered aircraft without extra paperwork. But when flying a UK registered aircraft in UK airspace, which is what I do most of the time, it's just created extra paperwork and changed the logo on the front of my license.
(3) No, the UK is an example of the most stringent adoption possible, others have either sat back to let others make the mistakes or adopted a much more pragmatic approach to implementation.
(4) No, it defeats the whole point. The only national PPLs were all ICAO compliant and so it was only a matter of a bit of paperwork for somebody to fly abroad or in another country. The UK NPPL and others seem likely to be sub-ICAO and thus we get a lack of cross-recognition. The solution to the high cost of the JAR PPL is to sort out that particular mess, not create another tower of babel worse than that which JAR-FCL was designed to solve.
(5) No idea, and frankly they've no interest in the PPL level of JAR-FCL anyway. At the PPL level, I think that the level of consultation on the content and implementation of JAR-FCL was woefully inadequate.
G