2 years from date of manufacture if the lid is sealed properly.
After the soldering process is completed while the pipe is still warm, you can wipe the joint with a dry clean rag to remove the excess flux. If the pipe has cooled or fluids are passing through it, you can use mild detergents (i.e. Dawn dish detergent) to wash off the excess flux. 
Yes. It is very important to stir upwards from the container bottom to insure any salts which have settled are mixed evenly into the flux. This should be done before the start of each project.
2 years from date of manufacture.
Less than 2%.
The only difference is that Safe-Flo is the retail branded version.
No. Oatey does not manufacture a solder or flux for this application.
Yes. Flux is designed as an oxidation remover and actually etches the pipe in preparation for the soldering process to be successful. This is why it is important to flush the lines when not using water soluble fluxes. Wipe excess flux off the exterior of the pipe after solder joint has been completed, and never wait more than four hours to solder a joint after applying any flux to the joint.
You will start off by getting a five gallon bucket and small electric pump with two washing machine hoses. Mix 1-one pound box of TSP (Tri-Sodium Phosphate), 16 ounces (2 cups) of bleach and hot water in the five gallon bucket. Drain enough water from your system to allow this mixture to be pumped into the water lines, and circulate throughout the system for one hour. The best location for hose connections is the hot and cold washing machine valves. Drain the system completely and repeat this process again. After the second application, you will want to flush the system out through each fixture for at least 15 minutes.  
The water soluble flux is less corrosive, and self-cleans on the interior of the pipe as water flows through the lines. The water soluble flux has a narrower temperature range when heating, but it works with all the solders.