Liquid could not have had any idea who George Sears was. If he had, he certainly would have mentioned him while he was spilling his guts about Les Enfants Terribles to Snake. He even explained how the Genome Army was connected, so I doubt he would have left out such a big factor as a third clone of Big Boss if he had known better.

As for the fidelity of Big Boss's remains, yes, genetic engineering technology in 2005 will certainly be more advanced and efficient than it was in the '70s, when Les Enfants Terribles was initiated and Solid and Liquid (and presumably Solidus, shortly afterward) were born.

Also, Big Boss didn't die until 1999, by Snake's hand.