one: I don't know that, I don't think it's possible, you have to select your desired palette each time you start GIMP
and two: I really don't know that
I don't need this feature so I didn't play with it much, I personally find it more easy to make a new layer under everything called "color palette" and put there near my drawing the color palettes I need and pick up the color a want when I want by checking the box "Sample merged" on the "Color pick-up" tool.
Welcome! Thank you, I have GIMP for a while but I didn't knew about palettes, I've found out by accident so I thought maybe some other people don't know about this either. I think I should fool around more with GIMP XD