Since you are fostering her, how is the adoption process handled? You've had her quite a long time . . . .

What about the old re-training route? Confine her to a smaller area, with a litter box.

Oh, that raises a question. Does she have a litter box in the kitchen? She sounds like she is afraid of dealing with the other cats, and she is marking the kitchen as her territory and all others keep out.

You certainly have patience to have tolerated this behavious for so long. Best wishes to both of you!