If the LL gave him a key for some reason wouldn't he have told the authorities by now?
I think your argument is definitive Pisces : VT was not in the flat when Joanna got home because he had no keys and the door was locked.
Either he was feeling randy, heard from CJ that she was to be alone, and rang the doorbell, or his story that she spontaneously invited him in is true.
But even if the attack was not premeditated, the death-dealing act was the culmination of a fight and he cannot have failed to realise before or during the strangulation that it was highly liable to result in her death or very serious injury. Everything else is just window-dressing.