Recommended settings for Specialized Brain with respect to durability I am seeking the right combination of shock pressure vs. Brain threshold settings for new Stumpjumper Elite (with Specialized Brain inertia valve) with respect to durability.

Are there any recommended settings for the trail/all-mountain kind of riding for the brain? 

I found this chart suggesting 3-open setting for the brain: 
http://service.specialized.com/collateral/ownersguide/new/assets/pdf/AC0419_2013_air_chart_11x17.pdf

Sure it is affected by personal preference. My concern is to find if there is any significant influence of the settings to the Brain durability/service interval by next extremes: 


low shock pressure + hard Brain threshold
vs. high shock pressure + soft Brain threshold


Does any of these extremes has significant influence on Brain durability/life-time? Or is it just a matter of personal preference?
The threshold setup of your shock should have a negligible if not non-existent bearing on its longevity. Run it how you like it. 

To give you an idea of what you're concerned about breaking, watch this video that describes how Brain shocks work. About 2/3rds of the way through you will get to see the internals of a Brain shock's inertia valve. They are not as delicate as you may think.