I have a problem with the Maple(Authorized Apple Service Centre)

Sir, I have a problem with the service provided by Maple(Authorized Apple Service Centre). I bought an IPhone 5s on Jan24th 2015. My phone is very much under warranty. I got a problem that after upgrading to one of the latest beta version of OS (The official one) my phone was not charging. Later, I then took my phone to the service center, they told they would downgrade the OS and everything would work fine I said OK and submitted the phone. Next day I received a SMS saying that my phone is repaired please collect. I collected immediately and it was working fine. But that same evening it again had a problem of and this time my phone was functioning abnormally. The calls messages were made automatically some random app would come up by itself and all those sorts of things were happening. I again took the phone to the same service center the next day. They took my phone again and after 5 days I got reply that your phone is rejected under Apples warranty terms and condition since some unauthorized modification has been done on the phone and your phone is now dead. They say that the some resistor near the battery is missing. I was shocked, why would I even give my phone that too IPhone somewhere outside when it is under warranty. I immediately went to the service center. They were like " Sir this is the decision from Apple we don't know anything go and contact Apple". I contacted Apple they agent took up my request and after 10 days he says Sir we have investigated and an unauthorized modification is done. Contact Maple. When I ask them they say contact Apple. I'm in the middle now and not able to understand what to do. I'm 100% sure that I did not have my phone repaired outside. Can I file a case against them. If yes, Can you please tell me what are the approx documents that would require to file a case.