Question #!. No, but maybe. I have electric pumps in a number of my cars (retrofit). Sometimes if the mechanical pump is in the correct position when the engine is turned off I will get fuel flowing through, other times not. I just think it depends on the lever position and therfore the valving position in the mechanical pump.
Question #2. Yes, but ... . If the car stalls, rolls over etc. the pump will still be pumping even though the car may not be burning fuel. This creates a fire hazzard. At the very least a separate shut off switch where the driver can reach it if needed. But that is limited to actually reaching it in an accident. Ford injected cars have a shut off shock switch that if the car rolls over or is stuck abruptly it will disengage. They are typically found in the trunk and wrecking yards sell them pretty cheap. Not sure what they cost new. Modern injected cars also shut off the power to the fuel pump if the engine stops turning for about 2 seconds. That is ECU controlled and on a carburetted Pinto not going to happen.
My own experience is if the electric pump has a low enough pressure it will just "assist" the merchanical pump and there are no issues. However, if the pressure is too high (darn those Holley Blue pumps!) I have had leaks at the mechanical pump. I do like having both as it becomes redundant security. But, on some cars I only have the electric.