Range: Every Go players.
Yes. (Honorific ranks put aside of course)
Pro ranks don’t go down, even if you get weaker.
I believe pro ranks are not a measure of strength, but of achievement. You get to a higher rank by winning tournaments or just winning games in general. Some older players are 9p simply because they’ve played a lot, while some new pros are 2p despite being incredibly strong, because they never won a major tournament and haven’t played enough games yet.
I thought pro ranks in Japan are given by participation in the Otai, a ranking tournament?
The Oteai was abolished 20 years ago.