1. Yes, but you should get powered risers if you plan to have lots of cards.
2. It won't reduce any KH/s
3. There's no issue to begin with (see #2)
4. They are plugged via a normal power cable to your PSU (usually a molex connector, although there are sata ones too).
Thanks
but if (2) and (3) are not an issue, then why would I want to get powered risers?
Risers are just required for fitting many cards on one motherboard, and then the mobo has difficulty supplying power to numerous cards so you need powered risers.