To answer some of the questions posed:
The earliest A.C. systems were frequency wild at 208v which was then rectified to provide 112V D.C. for the main power busses.
So why 400 Hz? Early inverters were mainly used to provide power supplies for radio and radar equipment that 'liked' to have that frequency. It then became a standard. We actually used 1600 Hz single phase inverters for one particular piece of electronic kit in common military use back in the fifties and sixties. 400 Hz can be fed at high current loads without significant line drop caused by power cable inductance. At higher frequencies like that 1600 Hz I mentioned, line drop due to inductance is unacceptably high at the current ratings need by the aircraft power systems. As with everything in engineering, it is a compromise. Nevertheless the generator output is generally regulated at 208V which equates to 200V at the bus bar.
Most of the more powerful motors on aircraft are three phase and have no need to vary their speed - hydraulic pumps, flap and slat drives, fuel pumps even powered flying contol units (PFCUs) Three phase synchronous motors are more efficient than non-synchronous types.
The 115V 400 Hz outlet in galleys is a power outlet. Most electrical accessories - especially heating units and the cleaners' vacuum cleaners - are not frequency sensitive and work well on any frequency, as long as the voltage is correct.
D.C. is often quoted as being Zero Hertz but tackle it by Fouriers analysis and you soon see that it contains an infinite number of harmonics. In other words, think of it as 'Infinity' Hz.