Most light aircraft went from 12to 24 volts in the 1970s as electrical systems were getting too heavy due to adding more features (radios, lights, heaters, sensors, etc.). The higher voltage allowed smaller, lighter wires and electrical equipment.
Adequate means that nothing will jam within the certificated flight envelope. A fixed number cannot be given because it depends on the general and local flexibility of the structure. You will need to show in tests or by deflection analyses that your clearances are adequate.
With regard to the initial question of engine placement, there is a simple thermodynamic question to be considered. Do you drag the airplane through the messy air that the prop makes, or do you drive the prop through the mess the airframe makes? It turns out that propellers, especially high...
