BootMgr is part of UEFI. If UEFI is still alive in the phone, you should also be able to flash. Try this:My GPT already destroyed on 520... and also destroyed SBL too, Stucked in LumiaBootMgr.. Still unable to get into Emergency Mode. I tried all several ways.. :/
- Start Windows Phone Internals tool
- Go to the "Flash" section
- Connect phone
- Press and hold Power and VolDown-buttons for at least 10 seconds to restart the phone.
If Windows Phone Internals detects the phone, it will try to force the phone in Flash-mode. From there on you can flash stock ROM or start unlock procedure.
Good luck!