This may help you understand how the GFCI protects. The GFCI must have the same neutral downstream for all the duplex outlets it protects. However any upstream connections to the neutral are irrelevant. Why? The GFCI has both the hot and Neutral go through an internal current transformer. With both both sides of the circuit the magnet flux will cancel and no current is introduced into the current transformer. However if either the hot or neutral has current flowing through it but not the other side the current transformer will pick this up. Once this current gets to a specified level it causes it to trip opening the circuit.
The life of a GFCI is around 10 years. Note that some older units may fail "closed," meaning they'll still conduct electricity, obviously a dangerous situation that defeats the whole purpose of CFGIs. The newer styles fail "open," meaning they'll no longer work, your circuit will be off but you will be safe.
On a final note, note that GFCI outlets will wear out, so you should test them about once per month. To test the outlet plug in a simple night light or other electrical device. The device should turn on, if not press the reset button it may have tripped. Then press the Test button. If the GFCI is working, the power will immediately be cut (your light will go off). Press Reset, and your device should turn on again. If so, the outlet is working as it should. If the light doesn't turn off when you press Test, you have a problem. The GFCI might be improperly installed or malfunctioning, and it won't protect you from shocks. You'll need to replace it or call a professional electrician for help.