Is it true using a lower gear when going downhill can reduce the risk of accidents due to brake failure?
Uphill, turn your front wheels toward where a curb should be. Set your parking brake. If the brakes fail, the wheels will turn your backend back toward the side of the road. If downhill, turn the wheels toward the curb/side of the road with parking brake on---if a failure, downhill gravity will take the car to the side rather than rolling into traffic.
There are a lot of things that you can do to reduce fuel consumption in your car, brake in plenty of time- do not brake harshly at the last second. Do not start aggressively, do not over rev the engine, try to build up speed when going downhill, because accelerating uphill burns more fuel and obviously travelling at faster speeds burns more fuel.
The self adjuster brake kit is replacement hardware for the adjuster for the brake shoes. If there is a problem with the adjuster it might allow the brakes to back off causing a brake failure. If you are changing this yourself be aware there is a left and right side, if these are mixed up or not installed correctly you will have a brake failure. Best to leave it to a qualified mechanic.
No, you will not have brake failure. However when the brakes are wet and you apply them, it takes a couple of seconds for the water on the pads and rotor to evaporate. During that time you do not have good braking. But the water itself will quickly dissipate due to the heat and friction of the brakes. Water does not damage a brake system.