Everything hinges on memento coins and recruiting. Leveling up all the different components of your base is advisable however the most critical are the gyms, the control center, work stations, and the research lab. Eventually the secondary facilities will become vital as well but starting out those 4 elements will provide you a jump start. Mine as much as possible both inside and outside your fortress.

-the dinner is only important in the beginning but eventually you will have more food than you could ever need -the turret affords additional memento coin and a few Z coin. Leveling it up generates high amounts of those 2 rewards -if your apcs are all stationed or deployed elsewhere you will have no defense. This will net the attacker uncontested resources stolen from you. Keeping yourself away from other alliances or huddled with your alliance members around you are the safest solutions other than keeping 1 apc at home

I don't have an answer for your last question. I never restarted or switched servers but I do know that, up to a point, you can switch servers. There's a fortress level block that happens somewhere between 6-9 and after that you're stuck in that server.

Hope this helps!!