Hello Dave. My paradigm of John and Kate involves possessing a bit of information on WHH.
Kate's final days would then involve meeting him and discussing terms. Final meeting in a pub where he lavishes liquor and "agrees" to terms. She is later gaoled for D & D.
When released, she is directed to Mitre sq.
Now, that place was notorious for solicitation. Her body is found there, "Hmm, know what she was up to." Perfect way to dovetail with the previous killings.
The knife wielder? Try the "notorious and unpopular character who was tete-a-tete with Hurlbert."
And it would be even better if this character were a "private investigator" who advertised about finding people and who specialised in divorce cases--and who also was known to provide "blackmail counseling."
Finally, it would help if this person were a known sociopathic thug who:
1. had an office close to Hurlbert's favourite club,
2. admitted to having killed a woman,
3. was eventually imprisoned and regarded by the prison grapevine as "Jack the Ripper",
4. were involved in planting evidence in the Stride killing to impede the investigation.