Joe - It's a good idea to try to follow the directions as closely as you reasonably can (at least the first time you do a particular kit) but even a kit wine is dependent on yeast, a living organism whose behavior is not entirely predictable. The expected date is only an estimate - go by the S.G. reading and bubbling. The point here is to wait until fermentation is finished; if your temperatures are a little below average, it may take the yeast longer to finish fermenting.
Adding the sorbate will prevent renewed fermentation after the first fermentation has finished. It does not kill the yeast, it will not stop an active fermentation. Rather, it prevents the yeast from multiplying, so that a few stray yeast cannot re-start fermentation after the original fermentation has finished and you have racked off the original yeast. So wait until it stops bubbling and the S.G. is down at or near the expected level, before you add the sorbate.
Personally, I do a lot of kits and only add the sorbate if I am sweetening the wine. If the fermentation has finished and there is no residual sugar in the wine, there is no need for the sorbate. On the other hand, it doesn't hurt.
I wouldn't worry about the SO2 levels -- follow the directions for this, too. You will probably bottle this wine in a few months -- if you keep the carboy topped up and add the recommended pot. meta. that comes with the kit, you should have no issues with oxidation. If this were a wine you intended to cellar for a decade or more, you might want to measure the SO2 levels right before bottling, and perhaps adjust them. In the case of a 4-week kit, you'll be lucky to have a bottle or two left by the end of the year, so don't worry about it. Enjoy!
Doug
