Help! A complete mess of merged ASIN and 'lost' high value inventory

I hope someone can shed some light on this because I feel like I am banging my head against a brick wall!

I am a brand owner on Amazon, I am the only seller of my own product. The last shipment I sent to FBA of one of my products was flagged as being mislabelled. When I looked at my listing somehow the EAN had changed, the FC managed to accept the stock by searching my product in their database and attaching it that way. How the EAN changed is a complete mystery - Amazon say they haven’t changed it and I haven’t changed it, I entered the EAN correctly on the listing and before this had 3 other shipments accepted into a FC without issues. The FC who received my product said before any more shipments will be allowed I had to fix the EAN problem on the listing.

So, I contacted Seller Support to see if they could change the EAN (they couldn’t), they told me to create a new listing with a new ASIN and they would merge/map everything, retaining all of the information, EBC, reviews etc and transfer the inventory. They did this and nothing was transferred across apart from the incorrect EAN, so I complained and a new person remerged everything and it was fine - everything showing correctly, inventory, reviews, EBC transferred. Then for some reason (I really don’t know why!), another Seller Support person came along and undid everything the previous person had done and the listing was a mess again with no inventory transferred. At this point, my inventory was showing under ‘stranded inventory’. I complained again to SS who told me to start again, create another listing with a new ASIN and new SKU, they would then merge/map the two ASIN’s, reinstate the stranded inventory and everything would be ok.

10 days down the line, my inventory is completely missing - it’s not under stranded inventory, I cannot find it anywhere. Bear in mind, this is 25 units which are worth approximately £5000. I got a reply from Brand Registry support earlier (I gave up on the general SS as they had already messed everything up) saying ‘We do not have any more the stock’. I have escalated the case, but I have no idea how to resolve this - I was told that my inventory could be transferred, it’s now missing and it’s very high value inventory. As part of the escalation, I have given the original shipment information (shipment ID, Amazon ID, FC it was shipped to and the date) in the hope that Brand Registry can find my stock.

I really don’t know what to do next - on occasion I have got someone who has known exactly what they are doing with this issue and it has been resolved, then someone else has come along and un-done everything they have done!

Has anyone else had a similar issue or any advice at all on what I can do to resolve this?