To finish off this thread, I was able to fix this though some trial and error experimenting.
I found that if i associated more than one extent with a single target ID, then neither extent would appear in the host.
So long as I maintained a 1-1 relationship between extent->target then all works well.
This was never the case before. I had multiple extents associated to a single target for the esxi host happily. Now I have to break them up in order to be discovered.
Also, the Device path not appearing in the edit pane does appear to still be an outstanding UI bug but not directly impacting functionality.