Even if there was a clear replication of how life started tomorrow then it would still be in very controlled circumstances which, as memory serves me, JWs have already referred to and dismissed as (a) still needing the right set of circumstances and (b) a big difference between sparking life in a cell and the diversity and complexity of life as we see it now. Basically they simply move the goal posts to where they see the the God of the Gaps fitting once more.
You only have to look at how they treat far more well established scientific theories around things like human anthropology, Egyptian archaeology and flood geology to see how easily they bat away the direct contradictions to their theology.
Cofty is right - the basic teaching on life-force does give them a theological issue should the science on generating life get to the point of removing all the gaps. The question is if science is likely to get to this point to the extent that it would cause a major theological problem for Witnesses before the JW organisation collapses in on itself anyway?