Hi Bert...I'm never gonna hear the end of this now, am I?? :-))
The ones I have made I've just chopped off 6 inches of a cable, and used 2 Panduit IDC sockets. So a pair of wire cutters and a pair of water pump pliers for the IDC sockets. Not a soldering iron in sight!!
I must admit, it's easier to just buy them, but being able to make them has got me out of trouble a few times...
On the question of the two unused pairs for 100Base-T, I have made - and you can buy - splitters that do exactly that - send two different ethernet signals down one cable. Handy when you have a PC and a printer in the same room and not enough cables. But as we move to Gig Ethernet I won't be able to use these tricks.
Oh, on the second question, in principle those 4 conductors dedicated to power in 100BASE-TX could have been used as a second Ethernet, yes. I mean, instead of providing power. But then when 1000BASE-T came along, there were no spare conductors. All eight copper wires are used for the one GigE link already. So as of now, 10/100/1000BASE-T cards have to be set up by default the same way.
We use POE a lot where I work. We just put Voip phones on and the phone gets network - and power - from the wall socket and passes the network on to the PC on the same desk. We have personal video conference units that also work with POE but so far no PCs, not even Laptops. I can see laptops going POE if they can get the power requirements down a bit - maybe non-hard disk types that work with the cloud, like Chromebooks? I've just been putting in a heap of wireless access points and those also work on POE - you just run a cat 6 cable to where you want them and they get power and network from it. It's also great for sensors, security cameras, etc.
I think one advantage is that you have one beefy power supply in the ethernet switch rather than lots of little ones for the individual devices. Better efficiency.. But then there's a fair bit of loss in the cables as well, so it's a case of "swings and roundabouts" I suppose.... I think POE works at up to 100V, so not huge currents...but ethernet cables weren't intended for power.
Yeah, you got it. There's no big deal here. From the beginning of time, well, almost, some telephone instruments got all they needed from ma bell, and others ALSO had to be plugged into your AC mains. The automakers simply want to expand their options in PoE, to save on copper cabling.