"Until the end of the phase" lasting effects wear off after "When the phase ends" interrupts can be triggered, but before "At the end of the phase" delayed effects resolve (see FAQ). So you can steal The Red Keep or The Wall and use it when the phase ends before returning it to your opponent. But you cannot steal Iron Mines and use them to save a character from being killed at the end of the phase by Tears of Lys.
You cannot take control of a unique card if you have another copy of that card in play (that you either control or own).
- You cannot take control of another Sea Bitch (see FAQ).