Re: Isabela not returning

You've got to have your friendship with Isabella quite high before she will come back. When she does, it's before you fight the Arishok.  She interrupts your discussion with him after he takes over the keep. Make sure you've given her the ship in a bottle, and teased her and flirted with her a lot. Do the quest to help Martin recover his goods, and take her along with you on that quest.