a) What does it mean by aftermarket?
Components suitable for the device, but not produced by the device manufacturer.
It gets confusing because Green Smoke doesn't physically make the batteries, but you have a choice of going OEM (Original Equipment Manufacturer), or after market (not original equipment, but compatible). In the case of GS, aftermarket batteries are the better solution.
b) What simple mods can be done to extend the life of the battery?
You can't really modify an existing unit with ease, even then, you're limited to the physics of the battery.