[ headcanons? did somebody say headcanons? :3 ]
After Isabela leaves, Fae is furious. She's at a point in her life where she's lost everything, and Isabela was one of the last friends she had left. Only she didn't die like the others, she left. At this point she teeters on the edge of "fuck Kirkwall"/buggering off, and murdering everything in sight. Thankfully for the city, she chooses to murder everything in sight.
She agrees to the Arishok's duel because if anyone is going to murder Isabela right then, it's gonna be her.
After Isabela's kiss, Fae's still furious. Only now, she's furious that she wants to forgive her. She wants to be angry, wants to hate her but it's far too painful both physically and mentally (for real all that pacing was half "pace through the pain" and half frustration).
Every time after that that Isabela leaves Kirkwall be it just to go on a runabout on her boat, or just time to herself, Fae resents it. It's easier to resent it than be terrified that she wont come back.