I got the solution to your problem.
My m12 was also stuck in boot screen after install some apk.
But let me tell you if it was not me give you this... you can sit and wait for anyone to help our problem
This firmware is only avaibable yet in chinese forums, no english info cant be found in anyplace.
Please download this 2 files:
Firmware M12 tablet:
RKTOOLS ENGLISH LANGUAGE
1. With the tablet shutdown press and hold "MENU" Key and only then connect USB cable to pc, wait 3 secs and release "Menu" key.
2. The windows system will detect new usb device, please correct windows version driver in unzip folder from RKTOOL. (i think this only works in 32bits versions)
3- After usb device install ok, run RKBatchTool.exe
4. Device should be detect "GREEN"
5. Open firmware img file.
6. Press Restore button.
7. Wait until tablet restart and starts restoring the system.
8. Wait until it finish and is done.
I hope this help you and others.
I have here another rooted firmware but i need to tested it first before post link.