Kind of. Someone else writes the scripts. Certain movies seem to have more input from Gosho than others. For example, Movie 1 (The Time-Bombed Skyscraper) and Movie 18 (Sniper from another Dimension) have Gosho much more heavily involved, and Movie 6 (The Phantom of Baker Street) much less so.
Not in manga canon, not yet I believe.
Gosho has taken movie stuff and made it manga canon, so this could be true.