VW T5 gearbox problem

Manual or auto? If it's an auto then the codes stored in the gearbox controller may give you hints. You should get all of the gears lit or flashing on the dash display if fault codes are stored. But that won't apply with a manual box, which has no controller (but now I read more carefully, it must be an auto, I think).

The theory about the CAN network might have some validity BUT the CAN network is very robust - it's a 2-wire network, and a single broken wire won't stop it from working. And if you have had a single broken wire for a while and now the second wire has broken, you should have been getting fault codes for ages beforehand.

If the drivetrain CAN network fails, you will see plenty of evidence of it. You don't say what year, but assuming it's pre-facelift, the ECU, the ABS controller and the gearbox controller will all log fault codes if the drivetrain CAN network goes AWOL. This will be immediately evident to the VW techs, unless they are complete idiots.

Did you check to see if the reverse lights come on while it's in reverse gear?

One problem with main dealers (VW or any other) is that their "technicians" are completely dependent on the fault being one that the specialists at VW in Germany have thought about. If (for example) it were a failed chassis electrical connection which meant that power to the reverse lights brings the whole of a control module's ground connection up to 12V (thus in effect disconnecting it from 12V power) they would probably consider that so unlikely as to never document it.

But, having just re-read, I think your van must be a facelift with a DSG auto. If so, I would want to see the diagnostic codes from the 'box - it's very complex, and it would not surprise me to hear that the box can command an engine stop under certain fault conditions. Again, you would see an odd gear selector display on the dash if that's what is happening.