Critical Strike does work with lightsabers despite the description. From what I understand, the bonuses from Master Critical and Keen stack but they don't multiply off each other. They multiply the original range and then add.
For instance, a base lightsaber will crit with a roll of 19 or 20 (or 10% of the time).
By itself, Keen will double the original range. This boils down to adding 10% (or 2) to the threat range.
Master Critical will quadruple the original range, adding 30% (or 6) to the threat range.
Used together, you get 50% chance of hitting a critical, original 10% plus Keen 10% plus Master Critical 30% (or 11-20).
Multiple Keens don't stack.