D. Rex
Magic Eight Ball
One common theme from stories like this is a mechanism to 'punish' the bound evil. You can coerce them into doing most anything you want when you absolutely need to, but the more you rely on dispensing punishment to get your way the more you sour the relationship, and the more grudging their help. Push too far and they'll rebel. Erembour is imprisoned inside you, but the ritual was never meant to seal her inside a person, and the binding is imperfect. It binds her mind (intellect, memories, etc) as much as her power, so she spends a lot of the day sleeping to husband her strength to fight the bonds when she's conscious. She doesn't enjoy thinking with a hobbled mind. Her dreams play out in your shadow, making it monstrous. While conscious, with considerable effort she can perceive and act upon the world around you. When you die she'll be sent back to Malfeas, but while you're alive she may yet figure out how to slip her bonds and go free, so she has a vested interest in keeping you alive. It is her nature as an emanation of the Ebon Dragon to escape all prisons - eventually. Nothing says she can do it quickly. This also avoids your one year time limit problem, unless you liked that aspect.
In the meantime, if you concentrate very hard you can fully bind her for a scene (at a cost of 1 WP and a -2 penalty to anything that requires concentration. That makes your shadow return to normal among other things.). You can also impose a punishment, the nature of which I can devise or you can offer suggestions, at will. Likely triggered by a spoken phrase or incantation. It's both deeply unpleasant and disruptive (super spray bottle for the cat), so it can be used to interrupt whatever she's doing. Similarly, for 1 WP you can temporarily weaken the bindings that hold her, giving her a much greater capacity to act for one scene.
Hazel was more into the practice of enticing those she summons rather than brute force rule. Or so she was with lesser summons. She did not mind offering her subjects a little bit extra in order to get better results. And would tend to reserve punishment for those that tried to take advantage of her generosity. As well as repeat business to those she favored, and possible other rewards.
While Erembour is by no means a lesser demon, Hazel would try to employ a similar strategy. She would rather give Erembour incentive to test the strength of the seal, but an amiable Erembour could find her prison more of a house arrest than maximum security confinement.
Hazel is not so sour about Erembpurs presence that she would go out of her way to make things miserable for her unless Erembour was being a butt.
But then again, Erembour might know of Hazel's declaration that she is considering finding a way to completely destroy the demon. So regardless of if she believes it or not, provoking Hazel too much does have consequences as opposed to simple cooperation, which might have the girl reconsider. Hazel didn't seal Erembour out of revenge or hatred of demons, but rather to keep her from causing trouble in creation until things are sorted out.
That said, I do better like the idea of Erembour speaking to Hazel in her head. Be it genuine or sassy commentary. And a bit of banter back and forth could be fun. Though a mechanic to punish and reward Erembour could come in handy.
I dont think Hazel will mind her shadow being in Erembour control all that much. Might even things its cool and theatrical. It might even be pretty amusing for Hazel to be doing one thing and her shadow doing something completely different Peter pan style. Hazel might even offer that sort of embodiment of her shadow to keep the peace.
As for the weirdness of the seal, I think would be fun to keep it a mystery of how inclined the seal made them become. Like if death would release her or cause something else to happen. Might give her another reason to keep Hazel alive, and another reason for Hazel to take care of the seal.
Also! I should point out that Hazel has another mutation from the wyld, an odd mouth on the tip of her tail. Now Hazel believes that there is a god that lives in there that she calls the Storyteller. A being which Erembour is sharing headspace with. Though if it actually exists or not is yet to be determined if its real or just a bit of wyld induced madness.. But it wouldn't be impossible for Erembour to use it.