Wouldn't using the duress finger result in some sort of destruction of evidence charge?

As with most duress mechanisms, one would have to judge whether using it was better or worse than not -- I suppose ideally one would have two, one of which simply reenabled the passcode and would have some deniability (this is effectively already possible by using a nonstandard finger, unless LE has witnessed previous successful unlocks and can identify the finger which should be used), and one which initiated a wipe of the phone.