Hot desking sounds like the way to go.
However, you may have an earlier version that does not have host desking. If the office x-623 is only used by her, build a new fwd path that has x623 as point1 and VM as point2, and assign to her remote phone ext (x-ABC). When she is not using x-623 (leaves for the day), have her put in DND, so calls will ring remote x-ABC, then go to MB-ABC. So system forwarding will replace manual forwarding. MB-ABC will only light x-ABC, but you can use remote message notification to have the MB call x-623 via an IC, or have it call a DID pointed to x-163, or even call her cell.