Dare I say there is a recently published book, "The Camden Town Murder Mystery" by David Barrat (that's me!) - self-published under the imprint of Orsam Books - which has a section dealing with Cornwell's book, showing a number of errors that she makes. It also shows that there were other suspects than Robert Wood.
The Rising Sun murder does point to Robert Wood, though, to a great extent, and yes I know there were other suspects, her defacto Bert Shaw for example.
However, that little drawing on the 'assignation' postcard, the obviously frantic hunt for it afterwards, Wood's liking for the company of prostitutes, his attempts afterwards to build an alibi by asking a male friend and Ruby Young to lie for him, all point to his being the probable murderer.
Ruby recognising Wood's writing on the card and the fact that he was seen drinking with Phyllis Dimmock at the Eagle pub are all pointers too, IMO. I agree with Miss Marple with regard to Marshall Hall. A modern jury would probably have convicted Wood.
I agree with other posters that Sickert was a man fascinated by murder. The Camden Town murder was a tremendous sensation, just as the Ripper Murders had been. He may well have felt compassion for a young woman so brutally murdered and this may well have drawn him to sketch and paint 'What shall we do for the Rent' and others
Sickert WAS the Ripper... he wrote many of the letters cited. The paper evidence as coming from Sickert's quire... you can't argue against this, at least, sensibly (yes, his wife had access to the paper too).
The question is: was Sickert also the killer?
There is no evidence that Sickert wrote any letters at all.
However, if he did, your argument is still flawed because there were many people who wrote multiple letters claiming to be JtR. These people are well documented by Neil Storey and others.