Since most of your questions involve a cruise on Princess, you really should be asking your questions on the board here for Princess, under Cruise Lines P-Z.
As far as luggage goes, you give your larger bags, those that would not fit through an airport security scanner, to the porters. Make sure your boarding documents are in purse or carryon, not in checked luggage. There is no one else to give them to. Yes, they anticipate a gratuity. But realize that they are longshoremen, union and work for the port. They are quite well paid, as these are coveted assignments that they bid on, and often go to the most senior that are the best paid.
As for what happens inside the terminal, I can’t answer as I have never sailed from Seattle. Both of my Alaska cruises were from Vancouver. Those questions are best answered over on the Princess board. EM