I know this phone is ancient at this point, but might anyone know which of the spring contacts are for the GPS antenna (if any)?

Went ahead and got one of the magnet docks. The metal plate behind the One's back cover (between the cover and the battery) does NOT interfere with NFC, or any other radio that I've tested.

So, with the NFC antenna being part of the cover instead of the body/battery, is it safe to assume that a ferrous metal plate (used with a magnetic dock) wouldn't interfere with NFC use?