It depends on what she was doing later that evening though, doesn't it Stan? For instance, if she was filling a kettle, using the taps full-blast in order to wash a few things, listening to the radio, 'wireless' as they called it in those days, in the sitting room, or doing anything away from the kitchen or front parlour she need not necessarily have noticed a knock on the door.
A conspiracy, with the aloof Wallace as a mastermind is an attractive scenario, ColdCase. However, I don't believe in conspiracies of this sort, I'm afraid.
Conspiracy to murder Andrew Borden (a very wealthy man) used to be discussed regularly on the Lizzie Borden forum I was on a few years ago. If two or three people plot to commit murder, nine times out of ten it unravels because one of them can't keep their mouth shut afterwards with a spouse, lover, sweetheart, mother, brother.
It would take an absolute miracle for two or more people to keep mum for decades following Wallace's death. And what motivation would there be? There's no evidence that Wallace hated or loathed his wife, and even if he did why should men who had no bond with him, no allegiance to him, risk their necks to commit the deed for him? (And in those days it would have been literally a neck risking event.)
After all, unless there was direct evidence in the way of a letter or a witness to their transactions all Wallace would have to do would be to deny it afterwards if the others were arrested. Wallace had no money to pay anyone off and would leave himself open to blackmail by his unscrupulous co-conspirators or anyone they'd told.