You should be able to flash the A256 using bcusb using the spi feature. You may have issues getting it work via ISP as there seems to be issues on the A256 with this process , I have one of these modems and am currently trying to figure this out. You can however remove the SPI and flash it direct using bcusb.
Yes the bcusb pinout is somewhat confusing regarding SI/SO , you will have no issues programming it at 5v , as it is within the recommended spec. I have previously created a script for the A256 , but at present i am not at home until the weekend , i will upload it then, but you should be able to program a full Backup without a script.
Try this one out , it is just set to read/write original firm at the moment , but it can be modified to load Hax if need be. You will notice it has the boot and Cfg as one file , do not worry about this too much as it will not cause an issue , if just a bootloader needs to be loaded we can adjust as needs be.
I've got my modems but lifes getting in the way...sigh
Yes i do have v1.2 how ever been using other dev boared and draw power from modem so voltage not really issue.
(using v13.0 not v13.5 untested by me)I could read firmware but not write it. Still with new update guess sorted.