I believe this to be true:
The Unocal plan was defunct long before 9/11, and Karzai never worked for them - the latter a myth started by Le Monde.
Actually, Clinton first proposed the gas pipeline across Afghanistan. Bush more or less endorsed the idea but supported the idea that the pipeline not cross Afghanistan but but laid in the gulf.
The Unocal plan was indeed defunct long before 9/11. The Argentines tried to resurrect it, also to no avail. Unocal spent several years schmoozing the Talibs hoping to interest them in the project, but what Unocal, or any investor, needed was peace and a smidgen of sanity and that was a bridge too far. So they moved on to more fertile soil elsewhere.
The Afghans, Paks, and Turkmenistan have all tried to interest someone, anyone, in building the pipeline since the war, but none of them can promise peace or sanity any more than could the Talibs. So there it rests probably for another decade.
I can't comment on Karzai's employment with Unocal, it wouldn't surprise me, what else is an educated, urbane, world-wise man in Talib Afghanistan going to do for a living; your choices would be pretty limited. Sell your services as a war-lord, smuggle opium, run guns, run errands for Saddam and Osama, or consult for Unocal. If Karzai wasn't consulting for Unocal he was presumably honorably engaged in one of the other pursuits.