Regarding the cash box, what do you think of this theory? A sneak thief removes the cash box whilst Julia is out of the room. He then takes out the notes and pockets them. However, Julia returns unexpectedly, catching him in the act. She calmly asks him to replace the cash box and await the return of her husband; this he agrees to do. Unfortunately for Julia, he changes his mind and decides instead to silence her, before making his getaway.
John G, very possible. It is not the scenario I depict in my reconstruction of Parry and Accomplice theories, but yours would work, too. Possibly the killer was asked to leave, changed his mind when in the hall, grabbed the mackintosh... There are many variations. I think the key aspect of the sneak-thievery is that it explains why the cashbox was replaced. This I cover in my book, of course, giving credit to Hussey, who first suggested it in his Parry theory (1972).
So, the two key theories for the cashbox replacement seem to be:
Sneak-thief who is caught just after the act;
Wallace staging a robbery and (arguably) making a mistake.