Yes those are the mounting holes
If you can get to the floor above the carrier you can put a bolt through the wheel and floor as shown. Then you don't need to worry about the spare wheel falling from a defective carrier, or being stolen.
Have gone through the post and read the troubles you had getting the right carrier fitted and the long list of recalls. Don't feel comfortable drilling through the floor board.
No reply from the dealer yet and i'm not hoping they will - if it was difficult to find online, i'm sure they would have difficulties too.
Also van wrecker hasn't got back about the wheel carrier. He did say they will part out a new model Ducato (in transit) and i'm curious if the wheel carriers in those would be different as i've seen some on eBay of wheel carriers that cover from like 2009 to 2018? Because of a different 'bell' for each 15" and 16" tyre, I will ask what size tyres on the Ducato.
NZ Post has a forward shipping service for deliveries within the UK so there is hope that I will be able to buy the right rims.