It isn't super critical that a reset is done, however it will prolong the battery life when done because the way the battery is charged changes based on how old the CEM believes it is. The older the battery, the more it is pulse charged to keep it healthy.
The brand new Varta battery in my 2.0D shows 12.57 volts in standby, while it fluctuates up and down with a max reading of 13.04 volts when the canbus is active (doors unlocked) and engine is off.
Like LOB, I wouldn't worry about a reading of 12.84 volts from the battery. The electronics in the car are rated to sustain voltages of 14.5 volts so they should be fine.
The CEM is notoriously vulnerable to corrosion either in the plugs or internally, although I'm sadly no expert so I can't offer any firm advice on what you should do from here to address the issues you're facing.
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