Q1: bookmarks trade. Players most commonly use this to sell expeditions. If you implement some kind of scam-free trusted system for expeditions trade, this will cover most cases of bookmark extractions.
Q2: 1 day, 7 days, 30 days, no expiry. IMO 4 hours expiration is too low. 1 day is enough for “it will expire before you know it”.
Q3: As mentioned in QoL suggestions, temporary bookmark can be used to automatically create quick “place where my ship was destroyed” location. It’s a common problem for new players. Old players can use this too to return back and exact revenge or try looting what is left on battlefield.
Also if you were to implement some kind of “quick bookmark” button in future it should have quick settings with expiration adjustment (via right-click or hover pop-up menu)