First, the best start is to do some reading and research on your own. Every major sailmaker has a website and plenty of information about sails. One smaller one that you might check for more general information on sails is here:
In a nutshell, it's hard to answer your questions as it's doubtful you have enough basic knowledge to understand the answer, but here...
What is the best material for blue-water sails? Canvas or nylon or something else?
Depends. Dacron (sorry, I didn't mean canvas) lasts longer, but it's shape is compromised earlier in it's overall life span.
What is the average life span on well maintained sails?
Depends. Latitude (read UV exposure) plays a huge role in the equation. The sun can drastically shorten the life of a sail aside from wind, general handling abuse, etc...
At what wind speed does a stout sail face the real threat of tearing?
Depends. A storm tri-sail, more wind than you want to imagine dealing with.
A light #1 genoa can be damaged (probably not torn though) in 15-18 kts... Each sail is built for a particular load/wind range. It's up to you to choose the right one for the conditions
I.E. worst track record (the sail that runs out of life first, again maybe the main or the job or another?)
I think you mean 'jib'. Typically, mains are build with heavier materials and will last nominally longer than many head sails. Many cruisers sail with terribly blown out sails. They get to their destinations, but often attribute poor boat performance to yacht design when new sails would open a world of love, wonder, and stunning realizations that sail quality really does make a huge difference in boat performance.
Is it recommended that you keep a spare sail for everything? I.E. Mainstay, jib, ect.
What's a mainstay? Spares? No. Just sails appropriate to your goals and sailing venue. Cruising and race sail inventories are very different creatures.