You did not kill a cell in the battery. I'm also not sure how they came up with that diagnosis unless they actually removed the fill caps to test each individual cell. The batteries they were selling you at best did not have proper cold cranking amps. A 35 amp difference is a lot. If they were previously loaded with acid, particularly at the factory they might have suffered sulfation on top of that. Some chargers do have desulfation capabilities, but at that point your battery is on the way out and should be replaced.
Don't be concerned with key on voltage in the low 12s. When you turn the key on you are also turning on the lights and ECU plus priming the fuel pump.
Do check the charging rate at the battery, 13.5-14.5 volts at 3K RPM. If you are undercharging you will eventually run the battery down. If you see less than 13.5 we will help you pinpoint the problem.
If you are using the bike on a regular basis there should be no need to keep it on a tender.
We're glad you're here. It's a great community .
I'll second that!