The SoC window is not 0 to 100%, it's might be eg. 15 - 75 % or something. That's also why you don't notice battery degradation, since the window might be moving (keeping constant energy limits for 0 and 100% charged).

For kWh capacity it's just a matter of counting the cells. Actual kWh will vary abit by measuring technique but I assume there is a way to drain the nominal Wh:s from them.