I also would not recommend using adapters for this I know cables are no fun to try to hide and place for good airflow etc but in this case I think its worth it to have a little more clutter and run things in the proper manor then use those adapters that I would never recommend. I'd treat that the same as saying hey your car should have synthetic oil but hey give it a try and put non synthetic in and see how long she goes! :)
I wouldn't really give that analogy since I know many cars that are supposed to take synthetic oil run perfectly fine on regular oil(of course they don't race their cars). Problems between synthetic and non-synthetic oils occur when you switch back and fourth between them especially later in an engine's life. I would say using a splitter is more like trying to use an extension cord made for a lamp on an air conditioner. Trying to force too much electricity through wires that aren't rated for it. If you want to help clean up clutter a bit try getting sleeved extensions for your pci-e plugs. They give you a little extra wire length but look really nice and allow you to hide the psu wire/plug behind the mobo tray.
Well, it puts more stress on the power, but, as long as you are not running it to its "peak" that it can deliver, as long as the extension is of good quality, it should be ok.
It simply puts more resistance on the wire giving the power, for most power supplies that are able to deal with it, it should be fine.
Coolermaster has made some excellent power supplies in the past, but have also made some really crappy ones as well. As long as the voltages do not spike to much, or the "noise" or ripple is within reason, its fine. I think the biggest thing to realize here, if the power stages are designed properly, it doesnt make a difference who makes it, however, if the inner component selection is dicey or downright crappy, then any extra stress, running it to its limit, or simply asking more then it can give well then you will have problems.
pci-e 6 pin is supposed to be able to handle 75w 8 pin 150 w, max overall that it "can" support is approx 250 and 368w on the 6 and 8 pin connectors however SPEC is 75 and 150w respective according to ATX spec. so if you are using dual 6 to single 8 or single 8 to dual 6 as long as the rail responsible for the power can handle the extra resistance, is giving the proper power I do not see a problem. Do make sure to not have any sharp bends or kinks though, extra resistance on top of the extra you put there may not end up being a good thing.
Molex to 6 or 8 pin I do not like, but extensions are a bit different, at least its the same connector being extended and not a connector being "modded" to do something it was not designed for in the first place.