I don't believe that the 144,000 are the ONLY ones chosen by God - and I definately no longer believe that they are interpreted in the same manner that the JW sees them.
But on a different note - as to the question about God knowing they would fall away. I don't believe he did know until people made that choice - or actions moving them to that choice. According to the bible, time itself was created by God. Which means, that he lives outside of the dimension of time. I see it like this - what if you could view an person's entire life in one snapshot rather than a time-line. As that person makes choices in their life you continue to view the entire snapshot as it dynamically changes. In this way - as the person made choices you could see immediately where that choice would lead them in 30 years. It would appear as if they were pre-destined when in fact it is just that you can see their entire life at once.
I heard it explained this way - watching a parade from the sidewalk you will see each section of the parade as it passes in a linear fashion. But someone in a skyscraper would see the entire parade at once.
As for Adam being perfect - yes he was, but that doesn't mean he couldn't choose to sin. If being perfect meant you CAN NOT sin then Jesus really didn't do anythign here on earth did he? The point is that Jesus made the choice NOT to sin. Being perfect doesn't mean being a robot with no free will - it means you have the choice to sin or not. Being imperfect we don't have a choice - we try, but we can't make it through our entire life without sinning - it's impossible for us.