I concur, draining it of power seems to be your last-resort action. Disconnecting the battery is the faster but riskier way to do that.

To access the device internals, first remove the front snap-on plastic bezel. That will expose the perimeter torx screws holding the case together. The bezel can become very brittle with age so great care must be exercised in its removal. If time isn't of the essence, then flattening the battery as you intend will work just as well. Good luck.

BTW, I located a North American-purchased nuvi 1200 with V6.30 fw. It had been unused for several years, so the battery was already fully depleted. Connected to a power source, it showed the wrong time but once it got a GPS fix the time self-corrected to the proper local time.