I understand your points and dilemmas, and wanted to share some info just in case. Many of my expat friends have their children in the public schools and have them follow after-school English classes to maintain and develop their English. In Gland, the Active English Club (they recently moved and changed name but you can find them here http://www.englishclubgland.ch ) has been teaching all levels for many years, and their reputation is excellent. I know families whose children were able to reintegrate without any difficulties into the British school system after having followed English classes at this private school. I also know some students who went to this English school alongside the Swiss state school and then continued on to do university in the US, at which they got excellent results in their English essay writing.
Good luck!