1 -- sure, as BC said, if you find slimy substance on the surface, that is no good. After a while, some degrees of discolouration will occur, if it is just slightly brown, you can still use them for just about anything. (Maybe not for salad, to be used raw, they should be nice and white... however I usually peel the external skin off for shroom salads, that will most likely take care of the slight browning.) If it starts to get dried and shriveled up, I would chop them up and use for sauces.
2. -- I have tried freezing them before, but the texture suffers a lot. They can still be used for sauces etc., they will let out lots of liquid along with their flavours, so this way the flavours will still remain in the sauce. As most of the frozen vegetables, do not defrost prior to cooking, cook them directly while they are frozen. (However, we so love shrooms and they are great with so many things, we hardly need to freeze them even if we buy a lot of them...
3. -- As I mentioned before, if they are not slimy and have a proper mushroom aroma (not mouldy), they should be okay.
4. -- In the fridge, they can last up to 2weeks, if they are bought fresh and stored properly. (As BC said, packing in plastic is not ideal, paper bag or cardboard carton should be used.)
We often buy a 2kg case from a market, when they have a good price. Even though there are two of us, this delicious versatile stuff have so many ways to be enjoyed we have no problem consuming all of them while they are still healthy. Another idea when you buy a bulk of them, sort of plan on how to use them by looking up some different shroom recipes that appeal to you!