I stand and tend to wander around the house while playing unplugged. With an amp running I can't wander as much as I don't currently have a wireless rig LOL. When I get tired I often sit back on a couch and play while watching TV. I stand when recording because I can watch the meters on my console when standing. Sometimes I sit at one of those stackable square-tube frame chairs in front of my desktop screen if I am watching a video and want to play along. I don't have a dedicated chair or stool to park when playing, it's most often just the couch.
Neither. I take a knee when I play the national anthem in support of all the guitar players who were told to stop playing or asked to leave guitar stores because they offered their rendition of Stairway to Heaven or Smoke on the Water.
I never sit while gigging, when practicing, it depends, if I am learning something new, often I will sit, because I feel I can focus on what I am trying to learn. once I learn it, I will continue practicing it standing, so I get used to playing as though I'm gigging. to each his own, but this works for me.