Question #1: pins 8 and 5 are for the relay coil and the other pins are for the switch. The way a relay works is that a certain voltage given to the coil will trigger a reed switch. This means that you can have a low voltage control a high voltage without being connected to the high voltage. Pins 8 and 5 are NOT connected to pins 2 and 4 in any way. They are two separate entities. However, the coil can control the switch without touching it. ( tell me if im confusing, i tried to make it simple, i think
Question #2: A relay can be given + or - on either pin. You can have pin 8 be + and 5 be - or pin 5 be + and pin 8 be -. Those are the only two setups possible for a relay to work. The diode in parallel will prevent backlash from the electromagnetic coils (i think) and will protect the microcontroller.
A diode blocks positive voltage on the black side of it. This means that positive voltage cannot flowthrough the black part of it. However negative voltage can, so this makes your relay be triggered from only one setup and not the other. This can be useful in R/C.
Hope I helped...