1. If you want to change employment, you will need to find another employer and he has to apply for a new LMO and you get a new work permit for him.
2. You can see what jobs are under the program at
http://www.aved.gov.bc.ca/provincialnominee/strategicoccupations/entrylevel.htm and yes, food and beverage server is on there
3. It says on that page, the same as the LMO or higher. The employer will not get an LMO unless he's offering market wage. HRSDC will decide what the market wage is. Employer has to be paying that or more to qualify.
4. Your wifes occupation does not really have to do with your PNP application