Having just read this I would be very suspicious of your current repairer. As previously advised you need to get the van to either a good independent specialist (who would have access to odis) or a vw van centre. Whilst I know vw are far from perfect I don’t think many manufacturers would be that helpful talking to a 3rd party repairer fitting 2nd hand parts. Technically you haven’t given loads of money to vw to fix the van without success but to your current repairer.