Two further things that testify against a killer other than Wallace in my view.
1. I think the fact Julia had a bad cold should be considered some more. Would she really let someone in while that under the weather? What pretense could a visitor known to her give that wouldn't be overriden by her saying she was too ill for company. Apparently she was ill enough that Neil Norbury the baker boy noted it and she said it was a touch of bronchitis.
If it was "Qualtrough" you could argue she would have let him in to clear up the "misunderstanding" as the "sneak thief" theory goes. Not wanting to let the commission slip thru her and William's fingers I guess...
2. Here is another thought that occurred to me though stemming from this... what time would "Qualtrough" in this scenario have arrived? If he was stalking out Wallace and saw him leave, it seems almost blatantly obvious he would have to come soon after that to give him as much time as possible before Wallace returned. (Also it hardly seems likely being right there he would just dawdle around for half an hour before trying to enter 29 wolverton.)
The theory that has been suggested as I encapsulated in the previous paragraph is that Julia would allow him in and then they would wait for William to return to clear up the misunderstanding about the address and more importantly get down to business.
But if Wallace had just left, and he almost had to have in this "Qualtrough as killer sneak thief" scenario, then obviously Julia would tell Qualtrough this and tell him to follow Wallace and/or head back to Menlove Gardens Area. It is not at all obvious or even likely in my view that she would invite him into her home for a cup of tea and to wait for hours (or that this is a conceivable plan that fake Qualtrough and co. could rely on in planning). Especially with Wallace having left probably just 2 minutes or so before!