Get into fastboot mode without root or hardware buttons
If you've had problems flashing your phone through adb, you should really consider the Windows XP installation route. I've had to look around for some compatible  drivers,as I've been experimenting using Windows XP and even Windows 8.1 with success using SP Flash tool. As one of the the answer given before in this thread, you don't need ADB to flash files to your phone SP Flash tools; just the required drivers and connect the phone to the computer with the battery removed.

You could try VMWARElinux to install Windows XP on with with required drivers/ files, etc. I can confirm VMWARE windows allows USB devices to connect to it, including this phone. I've flashed through it with success. Don't know if the Linux version does, though.

After lots of research and headaches I've finally managed to fully root my phone. I've posted my files so you can try it out here:

http://www.needrom.com/download/cubot-gt72-rooted-rom-twrp-recivery/

Best and luck and regards.