Re : the missing pizza theory.
My opinion is that JY was killed in her flat hence nothing else missing like door keys, boots etc. No way was she killed elsewhere and the perp goes back to plant the stuff - that's stuff of movies. Even if she popped next door in stocking feet then surely she'd take her door keys.
So what happened to the pizza?
Did the cold blooded killer take it for a snack? Unlikely, again stuff of movies.
But the killer did remove it for one of two reasons:
a) Perp arrives at JY's and gets invited in as he's known to JY. Something happens. In the fracas that ensued in the flat, the pizza, which may have been opened ready to go in the oven, got squashed and smeared onto the perp's clothes. Hence, perp gets rid of pizza so no forensic link to food found on his gear at a later date.
b) Perp meets JY on her way from bargain booze. JY Knows perp and invites perp back to hers for a drink - "not eaten yet, don't worry I'll get you a pizza from Tesco" (we know from text to MW that JY is up for some company and this would explain why it take 11 mins to go the 100 yards from bargain booze to the Tesco Express, why she didn't buy pizza at Waitrose and why she may have been smiling as she entered Tesco).
They go back to the flat, something happens and JY is dead. Perp panics, what links him to the crime scene .... the pizza that was bought for him. The perp symbolizes that by removing that, he's removing himself.
Unfortunately, the perp never factors in cctv and receipts!