Unhappy Very unusual situation with HTC One Mini - Any Help?

A friend brought a HTC One Mini in a very unusual situation to me. The phone bootloader is "Locked" and "S-ON". It only boots to fastboot usb mode only. The battery is still ok because I checked with a script that is used to charge in that mode when the battery is low; at least it still has about 3870mV, and the minimum is around 3680mV. The problem is that:

1. When I try to unlock so I can flash a custom TWRP recovery the unlock spalsh screen does not appear, although the unlock command says sucessful.
C:\fastboot-win>fastboot flash unlocktoken Unlock_code.bin
sending 'unlocktoken' (0 KB)...
OKAY [ 0.154s]
writing 'unlocktoken'...
(bootloader) unlock token check successfully
OKAY [ 0.006s]
finished. total time: 0.161s
2. I downloaded this stock file OTA_M4_UL_K44_SENSE55_MR_hTC_Asia_HK_3.10.708.4-2.12.708.3_release_351634.zip because it matches the OS version showing on the fastboot screen on the phone. When I try to flash it from fastboot I get this error:
C:\fastboot-win>fastboot flash zip OTA_M4_UL_K44_SENSE55_MR_hTC_Asia_HK_3.10.708.4-2.12.708.3_release_351634.zip
sending 'zip' (346477 KB)...
OKAY [ 16.826s]
writing 'zip'...
(bootloader) signature checking...
FAILED (remote: signature verify fail)
finished. total time: 49.466s
Can anyone please render any help? I can follow any instructions to the letter, just that I have never come across this kind of issue before.

Thanks in advance.
I have a MOTO X - XT1049 (GSM Unlocked) with talk, text and 3G data flashed to
And a Nokia Lumia 822 on MTN Nigeria
Current Phones
MOTO X - XT1049 & Nokia Lumia 822
Current ROM
Stock 4.4.4 & Windows Phone 8.1 Developer Preview