What saves bike rims when one rises before hitting small obstacles at full speed? When riding a road bike with thin tires over small obstacles such as tree roots, there is a risk of damaging the tires.
Cyclists argue that rising off the saddle will all but eliminate the risk of rim damage.
Why does rising save the rims? Do elbows and knees act as shock absorbers?
Here we completely ignore the comfort of the rider. The rider is rising not to spare his body from the impact, but to spare the rims. Why does rising make any difference? The biker did not yank the bike off the road, hence the total weight has remained the same.
 A: 
Here we completely ignore the comfort of the rider. The rider is rising not to spare his body from the impact, but to spare the rims.

Actually, it's not quite right to ignore the comfort of the rider. Think of this from the point of view of Newton's third law: if the rider's body doesn't get high impact, then it doesn't give as high an impact on the bike.
Then the question is, why does moving the body off the saddle reduce the impact on the body? It's because what we mean by impact here is the force: when sitting, the rider gets almost immediately the force from the ground, though the wheels, through the bike frame, and the saddle, handlebars and pedals. But when standing only on the pedals (and on the handlebars), legs and arms play the same role as shock absorbers in a car. 
The main part of the body, which has the most mass, is suspended on the shock absorbers, and gets little force transmitted through them. This, in turn, means less force being given back to the ground—again through the saddle along with handlebars and pedals, then frame and then wheels.
A: The answer to 2, is that the 70kg of the rider is decoupled from the bike mass which effectively pivots at the pedal crank, which is approximately midway between the wheels. Forces acting on either wheel simply pivot the 10kg of bicycle about the crank pivot, significantly reducing the effective moment of inertia.
If the crank was welded, not freely rotating, forces on the rider would be far higher as the mass of bike and rider would still be inertially coupled.
