I think you still don't have it boabab.
When you turn on a device, electrons flow in this order:
- from the negative post of the battery to the airframe
- along the airframe to the earthing post on the device
- through the device making it work
- along the wire to the switch
- through the switch, breakers, etc
- back to the positive terminal of the battery.
When the switch is off, no electrons will flow (even though the switch is on the "wrong side" of the device) as the circuit is not complete.
As you can see it is much easier to think of "positive electricity" doing the same route in reverse, as Beeline suggested.
Regarding your last question, it's a bit confusing: slip rings are not used "in lieu" of diodes, rather diodes have to be used with slip rings in an alternator (a generator, which has a commutator, provides direct current). The diodes are connected in a full bridge arrangement so that when the current reverses itself, the connection is also reversed so that the alternator produces current of the right polarity throughout each revolution.
Even if a half bridge was used, a capacitor would not be required - the battery serves the same purpose of "filling in" those parts of the cycle when the alternator is not producing useful power. In fact, running an alternator without the battery connected can cause serious damage as very high peak voltages will be produced.