find my iphone: how can the thief change the phone name?

Hello.

I have had 3 iphones stolen the last 5 months.

Finally I got a ping from one of the stolen iPhones. I could follow it for almost 2 months. Since I am in a country were the police wont do anything it does not help me that I have name/address of the thief.

But this strange thing happened last day/last ping I got from my stolen iPhone:

Somehow the device is removed from my "find my iphone" list and/or the device have changed name.

The device was locked with passcode/touch ID.

I have changed password AppleID 3 times since the phone was stolen (well: The police wanted my password/AppleID to track the phone. Somehow the thief got this information and I found out when my address book had 150 new entries from the thief).

I managed to change password and lock the phone remotely after that incident.

To bad I cant ask Apple any question.

How can a device be removed from my account? (I have not got any warnings that someone have logged into my iCloud account from a new device/location)

How can a thief change the name of a stolen iPhone that is locked with touchID/Password/passcode? (Iphone6). Is there still DFI attacks that can wipe the phones?

How can I check if my account is hacked?

Why cant I remotely detonate my iPhone? The battery would be a nice little bomb making the phone worthless?

(and F lazy Russian tourists police that just ask "what should we do" when I provide address/name/video of a person that have stolen stuff worth over 5000 dollars including iphones/Ipads)