Re: leaving JY in the flat after killing her, (GR said when he returned the flat was on the snib only). Does that mean VT left the front door open, or took JY's key to regain access to the flat. If he left the door open it might easily have blown shut. VT would have to take the key with him once he had taken JY body from the flat, dumped it in his flat, then returned & tidied/staged the scenes in JY's flat.
On leaving for the final time he would have to be happy with how everything looked, then put the key in JY's rucksack and close the front door using the handle (possibly using the sock to hide fingerprints.)
I was wondering about this too.
Tabak left Joanna's dead body in her flat. He then returned to move her. He turned off her oven and took the pizza she had bought.
So either he panicked and ran back to his flat leaving her flat door open. In which case he just got lucky that it hadn't blown shut once he decided to return to collect the body.
Or he knew he would need to be able to get back in. In which case, he either left the door on the latch and just pulled it shut or he rummaged around to find her keys or he already had a key to Flat 1