2. Force of will could mean anything. People with high wil could get a bonus to spellcasting because they try harder, to weapon skill because they are more determined...etc
I would say that the connection between willpower and fatigue is a lot more clear than the connection between willpower and "determination to be good at things." Yes, you can make the stretches you mentioned, but I don't think willpower factoring into how long you can fight before you can't anymore is a stretch.
Basically, I'm saying willpower is the ability to keep going when other people would quit. If that's not definitional, it is at least a fairly standard conception of willpower.
