Not a smart idea. A local police may lack a sophisticated expertise to hack encryption, there surely are policemen using iPhones with fingerprint scanner, which means they know how it works. You are likely to add obstruction of justice to your charge.
Well put. I would go further and say no biometric data can be considered a secure password.
The can compel you to use your finger, I don't think they can compel you to use the "correct" finger any more than you have to tell them the pin code to your phone. Of course, you will still suffer the consequences as they may hold you in contempt, but that does not mean you are guilty.
In reality, the police have at most 3 tries to get it right and you have 10 fingers. Plus, fingerprint readers are not fool proof. I think you can get away with it, especially if touch ID is only keyed to non-obvious fingers (any but thumb and index) but you use those anyway.
