OK. Apologies for the wait. I have no excuse, just life getting in the way of life.
So Sven eventually responded and essentially made things good by sending me a free replacement plus some peculiar German sweets, test implant and a HF tag. Having checked it over numerous times with my phone and Proxmark I was happy with the functions regarding sector 0 etc. At the end of that week I popped it in and all is now well again 
From what I can make out I think it was just pot luck with me getting a dud. Though I think that alone raises questions about QA and immediately reminds me of Enginerd’s thought on IAM potentially placing too much faith in some Asian factory instead of them checking the product. What I found most frustrating however was the weeks of waiting for replies. That much could definitely be improved.
I learned a lot from the whole experience though. My biggest take away was simply to be 200% certain the thing works properly before putting it under your skin! I definitely feel stupid for not having carried out that simple check. Also, now that I have gone through the process of removing an injectable I realise that it isn’t quite as bad as my initial thoughts. Now that the dust has settled, the idea of having to remove another one doesn’t phase me nearly as much.