Where you pay your social security premiums is governed by EU legislation, where you are liable for income tax is governed by bilateral double taxation treaties between countries.
Agree with that. So if you are based in France, you pay social security premiums in France, and there is no way around this. And if any airline or agency is not doing this, they are going against every rule in the new book.
Ryanair got fined €7 million for breaking this rule by the French. And if Ryan can get burned, then anyone can get burned.