hi
you have reasonable questions and I will try to give you reasonable answers but I have not flown GA in europe.
1. In the states, you can certainly be VFR above the undercast, just comply with VFR minimums.
a. IF you are VFR above the undercast and you have problems and must land right away, you have better have an option in mind. You must also rely on navigation methods that do not depend upon you see the ground below you or nearby your route of flight. (vor etc).
AS a practical matter I flew small single engine planes VFR (though IFR rated and equipped) over the foggiest parts of california . I flew with a chart on my knee and could figure out a gliding approach to nearby airports, whether I could see them at all or not. Vicinity and pray sort of thing. The engine never quit and I never had to do it in real life. (though I taught and tested the technique to my students)
I've also flown over the Sierra Nevada (peaks over 14k) at night in single engine planes and am here to type about it. But it is demanding and the "OUTS" are few and far between. Having a two engine plane with a single engine ceiling of only 5000' only adds time to the situation , but again the engine(s) kept running.
So, fly over the mist or fog, but think about what you are capable of handling just in case. Get radar following (or whatever you call it there) and if you lose it, get a vector to the clear or some airport even if not clear and hope you can let down safely.
I remember reading that our aviation antecedents would fly over the mountains of the eastern USA using a cigar as a timing device...when the cigar was finished, it was time to let down and hope for the best.
Good luck and always think, and then think even more!