I have to say I'm slightly bemused by the turn this discussion has taken. Why is anyone assuming that VT does not own a car? Because he rides a bike or commutes by bike and train? The Netherlands has a higher rate of car ownership than the UK but 27% of all journeys are made by bike. Even here most people who cycle regularly also own cars. But using the bike is healthier, more fun, cheaper, and often quicker.
To me there is a straightforward narrative which requires none of this fanciful complicity of CJ or any other car owner. If we simply assume that (1) VT was booked to fly to Schipol in the early hours of the 18th and (2) either owns a car or has the use of a shared car with TM, then none of this complex conspiracy is needed.
(A) TM is at a function at least until late evening which VT does not attend because he is leaving for the airport in the early hours (and might not want to attend anyway if it is TM's works Xmas function, for example)
(B) VT has a plausible reason to be driving a vehicle right up to his flat door and loading it with large bags and heading west across the Clifton Suspension Bridge in the early hours. (presumably why the police were not immediately hugely suspicious of his movements)
(C) VT is alone for the evening in the neighbouring flat to JY, and he knows that GR is out of town.
The exact mechanics of the murder itself need not concern us; there are any number of plausible scenarios whereby the murder occurred in either flat, and the flat was left as GR found it, possibly with a missing ski holdall and/or an attempted clean-up/cover-up. Once VT is in possession of JY's keys, he is free to come and go between the flats with little chance of being seen.
Finally the heavy snowfall causes VT's flight to be cancelled and he travels on the 19th instead, but he is still out of the country, and the body is concealed by the same snow, when the alarm is raised in the early hours of the 20th by GR and JY's parents.
All the above is consistent with what we do know from A&S police, and from the insights of posters like Nausicaa and Aneurin, and introduces the minimum of additional supposition. So where does the need to borrow a car, or for anyone else to be complicit, arise? Where is the need to move a body over a 12-foot wall to the rear or side of the property?
DISCLAIMER: The above speculation is entirely that, and makes no assumption about VT's guilt or otherwise, which will be properly determined in court.