According to the new campaign book, the Cobra is "the only unit that can destroy an enemy T7 with a single lucky shot." But can't the Fireball do this too? The fireball has 2/6 against T7, so a lucky shot can do 12 points of damage, enough to take one out. Or am I missing something here?

To give you a technical answer the fireball would need 2 hits to cause 12 damage. while the cobra would have a phaser of at least 1+/10+ vs armor 7 so 1 hit could cause 10 damage also just for lolz the steel rain is a 1 hit kill vs armor 7 as well so cobra is not the only one. also a lucky guy with a laser could very well score ten damage too.

The Petard Mortar can one shot any vehicle as well, can't it?

Yeah - sounds like the "fluff" doesn't really match the game mechanics - oh well, wouldn't be the first time (the "fast" Recon Boys come to mind.)