1. I'm not sure Ian will improve more by playing against 20 years old seven footer than against a 6'8" 25 years old american player. French league lack size but there are a lot of warriors and some talented centers.
2. Pau-Orthez too. It's maybe the best BB school in France with a great coaching staff. As an example, Johan Petro has learned to play BB with Pau-Orthez.
3. False, french coach are quite good. Diaw left Pau-Orthez when he was 20 years old : he was too young to be a major contributor. Putting Ian in a hostile environement is the best way to destroy him. I don't see a foreign team investing a lot of time for a player who will leave them when he will be ready to contribute. In France, he isn't just a talented big with nba potential : his team wait some production from him.
4. I don't know Sagadin enough to judge if he is the best but don't underrate franch staff. You have good coach too in Italy and Spain. Though games are still the best way to improve your level, pratices aren't enough. Mahinmi won't sign a big contract he can just sign a 2 years deal, european teams know that he will soon play in nba.
Anyway, Mahinmi will likely sign with a top french team and it's the right choice : it will be a smooth transition for the nba.