Well, we've committed break and enter, and are in possession of a great many Chaos artifacts. No way we come out of this looking good if we get caught. Etelka might have local watch on her side. On the other hand, we don't even know what's in the basement. If we were still fighting another chair monster and she turned up, we'd likely be killed. Mortus doesn't have much magic left either.
I'd say we either leave by the front door and move fast back to Suiddock, telling Freddie to run for it, or stay and fight. No point hiding in the basement if the key is for a room there, since once she finds it missing she'll go check there.
Alternatively... she wouldn't know how many of us are in her house. One of us could jump onto the boat and speed off, hoping that the boat gives chase.