Hi Talsmar,
1- The myth pet is a master 4 version of the fire pet. Accordingly, you can either use it or another pet (tank for example). Generally once people get the myth they don't use the tank again as its more efficient to snare-kill.
2- Ditto - forever until you die
3- Yep

Although the benefit of the 'aggressive' stance is debated
Key difference between it and your old fire pet is that the myth has hitpoints - accordingly it can take a few hits, either when soloing or from AEs in raids, so it is much more survivable... Once you get used to snare-killing when solo you will never look back...
Regarding the parse, remember that the pet is only a small portion of your dps, less than 1/3 to 1/4 depending on your gear choices and ability. Where you will see the biggest parse difference is on AE heavy fights, where the firepet would often have died or you would be constantly backing it off for bubble.
My 2 cents
Sherron
edit: remembering how to spell