Electric potential diagrams
1) If one were think about electric potential as analogous to gravitational potential, there should be a gradual decline from the high potential (+) to low potential (-). Why isn't that seem to be the case?


There is and you would see it if you were able to measure the voltage at various points along the bulb filament. The connecting wire has very low resistance in comparison so there will be little, if any, easily measurable drop along those.


  2) If in circuit A, the potential has already reached zero at point C, how can the charged particles continue their journey to the lowest potential (point D)?


Think more of a bicycle chain rather than individual disconnected links. When you stand on the pedal force is transmitted immediately even though the links take time to travel from the rear cog to the chain wheel. The electric field travels at a speed close to that of the speed of light but electron migration is measured in mm/hour.