And there you go. Two different opinions from two different people with two different ideas of what a car audio system should sound like.
Music is recorded in stereo (at least music on a CD) meaning two channel. SO technically speaking, your two channels up front are all that are necessary.
OEM comes with front and rear so that rear passengers can enjoy the tunes as well. With the size of those seats, I doubt you'll have many who actually sit back there.
Another reason you might want sound back there is just for rear fill. WHile it's the same sound in back and up front, some people just like to be emersed in the mix as opposed to being in front of it. Just depends on your preference.
As for using the existing speaker wire and mounting your crossover near the oem amp (in the boot), yes you can totally do that. The oem speaker wire is better than most, as it is even twisted pair speaker wire. And no you shouldn't need a crossover with a coax speaker, albeit some of today's top coax speakers come with one in case you wished to bi-amp them. You will need a diagram to decipher which wires to use coming in and out of the amp, as for that, here you go.