1. Each bunch is usually 90g (3.2oz) of dried noodles. When cooked, it expands in volume. Depending on how hungry you are, some people might find that a bunch of soba per person is a bit too much. If left over, you can add the soba in clear soup or make salad!
2. You could serve the dipping sauce warm if you like. Some people in Japan eat zaru soba with warm dipping sauce. But I personally like it cool otherwise it seems to defeat the whole purpose of having cold noodles.
3. If you don't use yaki nori, then call it mori soba.