I will have a go - dont quote me
1. FL130 in a G reg, but this is not mandatory,
2. At its simplest aircraft can be individually interrogated, which is not possible with mode C. The response will include the aircrafts unique identity. Enhanced mode S also provides other flight parameters (speed, heading etc., but rarely applicable to units installed in GA aircraft.
3. Yes, the position is mixed across Europe and some areas even require mode S to be turned off now.
4. Correct. There are few reliable sources. Many use meteoblue which provides a pretty good service and form 215/415 is helpful as well as SigWx
5. NATS provides route briefs - you need to register which is free. There are various third party offerings such as Notamplot - some are free, some charge. Do a search on here - there is a good route planning software which a forumite has developed.
6a. No changes there unless clear of everything - 50 feet over the sea would be strictly legal as long as their are no swimmers or divers just below the surface or 500 foot away laterally,
6b. I have never heard that one.
7. Are you talking about coming off airways or in airways. I assume the former?
8. Any country can give permission to a permit aircraft if they wish and most do. Broadly permit aicraft are certified in a different way and to a different standard.
9. To avoid your position being displayed to other aircraft with TAS while not air borne. Many more recent units will suppress their squawk until the aircraft is airborne even if left switched on with Alt.
All of these answers could be expanded greatly. I have had a go from memory so more than happy to be corrected.