The tender is the best charger for your battery. Especially sealed batteries; the fewer amps you charge the battery with the better.
Do you have another meter you can double check numbers with? 10 volts is indicative of a failed cell and should not be enough to illuminate lights. Again, 12.0 volts is a very dead battery. Less than 12 volts usually indicates total battery failure. So while it's not impossible, I'm surprised anything came on with 10 volts and wonder if there isn't an issue with your meter. I mean it IS possible, but unusual. And, again, 10 volts is indicative that something is wrong with the battery, not that it's discharged. Your battery has 6 2 volt cells, 2 volts being the nominal voltage when it's discharged. So, together, they create 12 volts when discharged, 12.6 or so when charged (as you know). Less than 12, especially 10 or less, indicated one of those cells has failed. Occasionally you'll even see lead acid batteries listed as "6S", the 6 standing for the number of cells and the "S" referring to the cells being in series. To add to the complexity, you can sometimes find "6S2P" batteries which are 12 cells, made up of two parallel 6 cell setups. Pretty rare and mostly for specific applications (most lead acid batteries just use bigger cells) but those batteries have greater capacity.
Was the regulator/rectifier unit checked after replacing the stator? There's a sticky in the 900 section outlining how to test this.
In your fuse-checks, was the bikes main fuse checked?
Once you get it charged up, test charging voltage and report back. Although, despite it only being 3 months old, the battery is suspect here because charging system problems shouldn't cause a battery to become discharged overnight. However, a bad regulator/rectifier unit can certainly damage a battery. A common issue is someone having a charging system problem and having the knee-jerk reaction of replacing the stator without checking other components. And it becomes an endless cycle of replacing parts because one part destroys another part; until someone finally breaks down and diagnosis, and replaces, ALL affected parts.