1. It depends on how much the bike is ridden and how (weight, terrain.) If the bike is never ridden the wheel will last forever. But an improperly tensioned wheel will wear faster than one that is even and correctly tensioned. A wheel is s dynamic system that flexes and shares the load. No one spoke ever is carrying all the load and stress on the wheel. If one spoke is way out of range then the wheel is going to be flexing more at that point and the more if flexes the quicker it will fatigue.

2. Properly tensioned wheels will wear slower. But spokes don't "heal." Fatigue does not go away. You can slow down the process but damage done in the past will still be there. A wheel can have 70% of it's life used up in a year or so of poor tension and if the wheel is worked on and set up correctly it'll still only have 30% of it's theoretical lifespan but that last 30% can be stretched out by slowing down the damage to the absolute minimal amount of wear and degradation. Maybe another couple of years can be milked out of the wheel but never as much as that wheel had potential too had that initial period of neglect been avoided early on.

C. Rear wheels wear out faster because they are exposed to a greater load. The weight bias of bicycles puts more weight on the rear wheel and many people put more load on the rear wheel by loading cargo or kids out back. Plus there is also drive stress on the rear wheel. To make matters worse most modern derailleur bikes have dished wheels putting even more stress on the drive side due to all those silly gears in the way of proper even spoke angles. Having properly tensioned spokes in the rear wheel is even more important.