Here are the answers according to the order of your questions
1. Islam does not directly commented on oral sex. So, it is between a husband and a wife. If they want to do it, it is upto them. However, if there are infections in either of their private parts, then it could be transmitted to mouth during oral sex.
2. It does not matter whether you eat or not eat after having sex.
3. One may find it uncomfortable to bath after having sex, but there is nothing wrong medically or according to Islam if someone prefers to have the bath right after sex.
Hope this answers your questions