1st:Is an extra catalyst needed if making it dual? No, you can run a off road mid pipe with no cats more power but in some states you wont pass emissions
2nd:X or H? H=classic muscle car sound and Low end TQ X= high pitch sound gives more of a bite and its give 1-2 more HP then the H and its top end TQ so you will feel a difference at 3,000 rpm and x pipes sound better with a round muffler like SLP, Magnapacks, Borla stingers, or pype bombs. ( I run a off road x with SLP laoudmouth cat back I love it!)
3rd:what does catback mean? Cat back means the exhaust connects from the cats back any cat back system passes emissions
4th:best brand? Top brands out the are Borla, Magnaflow, Bassani it's all what sound preferences you like
5th:will i have to change headers? No, but for better sound and power long tubes are the way to go, unless you plan on turbo, the JBA shorties are the best way to go and there the only shorties that pass emissions.
6th:will i need to add an O2 sensor? No but if you run a off road pipe I suggest you get MIL eliminators($80) or a tuner ($375) to keep the check engine
light off. Also for some mid pipes o2 sensors extensions will be need for the rear 2 o2. ( I run a off road X-pipe from pypes and I needed o2 sensors extensions and I bought the MIL eliminators also)