#1
PS/2 is the only way* to get N-KRO. Like #2 the CM Storm Quickfire should have a PS/2 adapter included (mine did too). If you can't find it just buy one.
*there are strange and complicated ways to get N-KRO but they are only suited for connecting a N-KRO cabable keyboard with PS/2 or another N-KRO cabable connector to USB with an active adapter or a teensy to keep the N-KRO on USB. Unless your motherboard doesn't have a PS/2 port there's no reason to go with this complicated and more expensive method.
#6
No. Although you can do stuff with drivers to get more than 6-KRO, you can get for example 12-KRO by pretending that you connected a hub with 2 keyboards, or you can use a on-board controller within the keyboard (I've seen 64-KRO over USB).
None of these are N-KRO however. The difference is negligible because most people are limited to 10 (give or take a few) keypresses by their number of fingers anyway.
The main difference is that USB uses polling which adds a delay (<=1 ms for USB 2.0, less for USB 3.0 but there are no keyboards that use it yet) and the limit to 6-KRO** whereas PS/2 uses near instantaneous interrupts*** with true N-KRO (unlimited! keypresses).
**Although it can be circumvented (see above) this will add further delay, depending on the implementation either always or just when more than 6 keys are pressed.
***We're talking about a few CPU clockcycles delay. Definitely below 1µs though, even for the worst case.
EDIT: fixed a(n) mistake