I don’t think it matters if the plates are fake or not. If there are images or footage (Malaga video) that show the VW with the plates that match the Alcossebre sighting then that’s enough.
In terms of risk, there are two points worth considering: if CB had been in hiding in the Algarve, it would be worth taking a risk to get away from the heat there, also CB has a significant tolerance for risk.
This scenario, if true, would mean that MM went north of Alcossebre, Germany would be the obvious choice, but she didn’t return with CB in the Tiffen. Unless of course his comment to DF was true because it was underway!
All sounds far fetched but who knows.