1. No!! If you are going to prewash, wash the whole pieces of cloth before cutting. If you wash the top after piecing, it will ravel like crazy. Just think of all those raw edges!
2. Sorry don't have an answer for this one.
3. Many people use unwashed and prewashed fabrics in the same top and have success. If you are in a swap, unless it is specifically stated to use one or the other, make your block to the correct size and send it off. Making the block larger and washing will have the same raveling problems and I cannot see how you could cut it to the correct size later.