mtkflash in real DOS should work.
Inside of the zip file is GCE-8525B103(W).exe, this file is actually a compressed archive itself, extract it using WinZip or WinRAR and inside you will see a file called IDE.hex
Grab some version of mtkflash
and attempt to backup your firmware. If you can successfully make a backup, then try flashing the firmware with mtkflash.
mtkflash can handle firmware images in both hexadecimal and binary format. Firmwares are most often found as binaries, but this one is in hex. Be sure to specify that the file is in hex format using the '/h' flag if using version 1.80.1 of mtkflash. All other versions of mtkflash expect hex files by default, so you should not use the '/h' flag for versions other than 1.80.1
You could also attempt to use MTK WinFlash
with the ide.hex file, but if the official Windows Flasher failed, then I doubt MWF would have any better luck.