I have a load of European trip writeups
here if you want to see how good you have it over there
Why do the UK controllers object to "pop-up" clearances? Here, controllers very rarely decline, believing (accurately, I think) that having more of the existing traffic "in the system" improves their ability to manage safety.
The ability to handle popups varies around Europe. A few countries do it OK; I believe Sweden and Germany are normally OK. Most others don't like it, or the clearance takes so long that unless you can hover you will run out of time. A few pilots I know have killed themselves while waiting for a popup clearance, although obviously that was just a factor; they should have executed Plan B sooner. All had IRs but all did not have oxygen and that limits your options a lot.
I think ultimately it is to do with ATC funding. Here in the UK, there is a virtually watertight separation between the "controlled airspace IFR" controllers and the ones which handle the mostly-VFR GA down below. The former is funded from the IFR route charges (paid by all a/c above 1999kg and quite substantial; a significant % of the cost of fuel). The latter is funded by a mixture of sources, some military-related and some CAA and, perhaps cynically, if it was not for the hundreds of serious CAS busts each year, much of this funding would dry up. All we would get is the ICAO FIS obligation which in the UK is a VFR non-radar service staffed by non-ATCO personnel. Transiting upwards between the two systems is usually impossible. Cancelling IFR and going down is usually OK but usually not what you want to do if the wx is bad
What ever happened to the Euro restrictions on N-reg aircraft and the use of US instrument ratings?
In short, aircraft flight, maintenance and long term parking are not affected. So you can keep your N-reg, maintain it to Part 91 (A&P etc as usual) and park it anywhere. Denmark and Norway are two places where regulatory attacks (fines) have been reported on long term parked N-regs but details are scarce.
What the EU did was to force the pilot(s) of any aircraft whose "operator" is EU-based (these terms have not been defined anywhere, hey ho) to obtain Euro papers. So, flying an N-reg, I have to have the FAA PPL/CPL/IR + medical (State of Registry requirement) plus the Euro equivalents which I did in 2012.
This dual papers requirement has been repeatedly postponed and now stands at April 2016.
There are conversion routes from ICAO IRs (say the FAA one) to the Euro IR. Currently this is just an oral + checkride, plus of course the Euro medical with its tighter audiogram requirement for the IR (both ears need to be good).
Summary
here