I never hesitate to taste wine (or beer or mead) at any step of the process - you just need to keep its youth in mind in judging what you taste. You can get a very good idea of the sweet/acid balance, as long as you can put aside the funky other flavors that will go away with aging. I don't like my fruit wines above ~1.020, no matter how acidic the fruit (black currants can be pretty extreme, blackberries slightly less so).
The answer for the sorbate is probably not. It is advertised as preventing re-fermentation, not stopping active fermentation. I have seen some people refer to stopping fermentation. Perhaps there is a chemical that does that, but it isn't sorbate.