Cavitation is the formation of vapor bubbles in the fluid stream due to a drop below the vapor pressure of the fluid. The formation of the bubbles typically occurs just in front of the impeller eye since this is typically the lowest pressure in the system. The bubbles subsequently collapse downstream as they enter a region of higher pressure. The bubble collapse is what causes the damage to the pump impeller.
No, most pumps which are designated as self-priming have a lift potential of 20-25FT vertical or its equivalent. It is a matter of atmospheric pressure (14.7psi at sea level) pushing on the surface of the water and therefore into the pump. The suction lift potential is controlled by Mother Nature, not the pump motor size.
No, bigger is better. Since all hose/pipe has friction loss (resistance to flow) based on liquid velocity and hose/pipe size, bigger is better. Just like electricity in a wire. There are common, published friction loss charts for various hose/pipe sizes. Often the friction loss factor is provided at a given flow (gpm) per 100FT of pipe length. Since the loss is exponential, the more you try to push through the pipe the significantly higher this loss becomes.
Yes, a Self-Priming pump and in particular Centrifugal types, need a volume of water in their casing to initiate the self-priming action. Once primed they should hold prime (liquid in casing) and start-up just fine when re-started.
No, the Max values provided are both at opposite ends of the pump performance curve. They are simply a guide for someone familiar with pumps to quickly assess the suitability of a pump size for an application. The Max Head / Pressure is at zero (0) flow and Max Flow rate is at zero (0) head or very close to it. You need to look at the pumps performance chart or curve and look at a particular flow rate and pressure you need.

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