Some input on the voltage/frequency/current questions
Voltage
Usa does generally use 115v 60hz A.C. but has other voltages 220etc available in some areas. It is correct that the lower voltage is "safer" in similar conditions than say the european standard 230v. it is also down to the design of the transformers as in distrubution systems all voltages are referenced to earth. The neutral generally being connected to earth.
In many areas the earth connection is actually in the center of the transformer winding so halving the voltage to earth this in the UK is particularly on building sites where 230/110 volt transformers are used - so you get 50v from either output wire to earth but 110v between. Some U.S. systems use this.
The U.K. power system was designed for small numbers of large substations feeding an area so the higher voltage (240v) was used to reduce voltage drop problems on longer runs, though Blackpool for one originally was a 110v system. The U.S. system uses a mid level (possibly 1000v distrubution with smaller transformers feeding individual or small groups of properties.
Both work for them.
DC undersea cables i.e. France to U.K. is because France uses 60Hz frequency & U.K. 50 Hz - try connecting them together and it is BANG Big BANG. also losses in D.C are lower at Higher currents.
Swiss railways vwith 16 2/3 Hz was simply that before the invention of clever electronic kit A.C. motors were virtually impossible to speed control. It was found that by using that frequency D.C. controls & motors could be used thus saving the expense of costly A.C. to D.C. rotary convertors or latterly massive valve types.
3phase motors are much simpler as a single phase motor needs mulit sets of ciols and a capacitor to start it. the simplest 3 phase motors need 3 static coils and an aluminium (usually) cage that spins, the 3 pase sets up a rotating magnetic field, this is induced into the cage and it spins.
Sorry can'y help on the 400hz bit unless it is to do with connecting alternators in parrallel - but I don'y know if that is done on a/c
Sorry thats long & hope it helps