My instinct has been and continues to be that it's most likely a failing chip.

This is where it's at:
  • MSM is unreliable, transitory at best
  • It's not seeing an SD card in a normal and reliable way to load anything from it using TXT commands
  • Pre-boot is available and is in fact its current default mode
  • Flashing with a modded ramloader RGN has somewhat restored it

The undisputable facts are that a stalled boot with "System Software Missing" message can be caused by one of only two basic problems - (i) a bad chip; or (ii) corruption of data in an essential region. Usually that corruption's in rgn14 (fw) or, if it's severely corrupt, perhaps in rgn41 (nonvol). The other cause as said is a physically failing chip but very often when that happens pre-boot isn't available at all. So, the only reason i was prepared to go down this rabbit-hole at all is because you have pre-boot, thus leaving some feint hope of a complicated "soft" failure.

Region 14 is certainly corrupt in your 78. When the unit boots, the "fw_all" data in 14 is involved and the device cannot get very far into a boot-cycle without that data fully working, definitely not far enough to recognise any inserted card. The modded ramloader i supplied bypasses that involvement to allow a card to be seen and for SD commands to be executed, however hasn't yet worked for you. You should try that again, but this time add an empty "update.log" file to see if that helps. I've seen a few devices over the years that wouldn't flash from a card without that file in place. It is worth a try anyway. Extract this ZIP and add the LOG file: [Only registered and activated users can see links. ]. The only files in 1113 folder should be the original update.txt and the added empty update.log. Then try to dump rgn41 again using Updater.exe and the modded bootloader RGN file.