Parallel external supply to CR2450 coin battery
Personally, I'd cut the +3.2V trace leading from the battery, install a shotkkydiode bridging the gap, then simply connect the jack for supplying +3.3V external power past the diode.

After this, you will either:


Have a working device whose life can be extended fairly indefinitely using external power.
-or-
Find out that there was stored program code in volatile memory, and know that more extreme (and potentially dangerous) measures will be needed if you want to continue to pursue this project.