If the ground of the system actually wasn't connected to ground at all as you suggest possible, whence would the current loop be completed?
I'm no electrician, just a sound guy, but I'll give it a crack...
I -think- that if the mixer's ground is properly connected, that safety ground also protects the microphone. Like a power cord, a microphone cord contains three conductors, and they are hot, neutral, and ground. So normally the unused phantom power returns to the mixer via the ground wire, and this would be a shorter path to ground than through a person. If this path is removed, it's looking for a path to ground, and a person holding the microphone and standing in water, complete with metal drains and/or lights, heaters, etc., may become the shortest path to ground.
As for having phantom power on without needing it, that is entirely possible. If they run a "contemporary" service (full band), though, it is likely that they are using at least one or two condenser microphones that require phantom power.
While looking for their web site, I did see a listing for the church which showed it with a contemporary service.
0.1 amps is all it takes to have a fatal electric accident. The current flowing through the body at 0.1 amps is generally more of concern than the voltage, although if provided sufficient voltage differential, the current maybe implied.