Sony Xperia SP- Soft-bricked, can't get PC to recognize
Lent my Sony Xperia SP (C5306) to someone, it came back soft-bricked. I assume this, because the phone vibrates 3 times when charging now, and the screen will not ever turn on at all. Sometimes the RED led at the bottom lights up or blinks twice, but never any other color.
I tried installing various older & newer versions of Sony's "PC Companion", to get the drivers working, but there's no hope there. Stupid Sony insists on updating it or it won't work, and the update always fails on every computer I've tried installing it. I tried various versions of Flashtool, including the older 0.9.18.6, as well as various versions of its driver package, where I install the "Viksam motherboard" driver for the SP (since there is otherwise no SP specific driver here).
Flashtool gives an error saying "This bundle is not valid -- cannot identify your device". It continues anyway, to the screen asking to put the phone in fastboot mode. But this is where I can not get it to proceed and recognize the device, using the Vol. Down method (Vol. Up doesn't do anything either). Sometimes, I can trigger a response by hitting the reset+power buttons on the SP. Then, it'll say device is connected, but debugging is off, and the it will disconnect the device and I'm back at square one.
I tried all this on Windows 7, then moved to a Windows XP computer, hoping to have more luck with that system. On XP, I installed ADB and its drivers, specifically Windows drivers for the Xperia SP that I found. But I can't install those SP drivers on Windows, and Windows won't recognize the phone is connected in the Hardware Device Manager. On the Windows 7 system, at one certain point (where I was hitting reset + power buttons), Windows did recognize the phone was connected, and it installed USB drivers for it. Yet that did nothing to help Flashtools recognize the phone.
Since my computers won't recognize the phone, and the screen does not come on, I do not know any way to turn on USB debugging. I'm not even sure its necessary. I tried flashing just the kernel alone, and this is supposed to not require USB debugging on. But it still doesn't work, because why else, the computer and/or Flashtool won't fully recognize the phone.
I don't know if the bootloader is unlocked or not, but the phone was rooted and SIM card unlocked. Is the phone soft-bricked, hard-bricked, repairable...?