What happens in the case of FBA as a refund has been initiated by Amazon, how can I see if the item is back in my inventory or not, maybe the customer received yet claimed not received !
In that case I lost money plus item both?
The best way to handle this at scale is a service like getida or seller investigators. I use both.
Yes, that happens.
Can’t I open case with Amazon to ask for money or any way I can check in my account item is back? I doubt, the customer received it but saying not received.
You can check with them. Amazon is here to make the customer happy, sometimes at your expense.