Thank you for the reply Mark. Yes, I will be replacing the battery today. Over the course of this weekend I've planned to get the car up on jack stands and check the fluid levels and the state of the connector sleeve.
RE the fault codes:
With the battery disconnected, I removed the rear wheels and cleaned both sensors (cleaned with electronic cleaner, applied dielectric grease to connectors etc) and the reluctor rings (wire brushed all the grease and dirt off after spraying with some de-greaser, noted no damage to any of the cogs). Reconnected the battery, managed to clear the U1041 codes (instead of clicking on the sub-menu or error description, clicking on the main module description brought up the "clear code eraser thingy", drove the car for two ignition cycles (the transmission was in limp mode - 2nd gear the whole time). Rechecked and now the only faults now remaining are the ECM ERR0xC (lost communication) and the P0715 Input speed sensor from the TCM. When using one of the live data logging modes, the TCM value simply shows "no communication".
I've checked pretty much everything else using IDS such as the transmission range sensor (which works fine), brake pedal switch works fine. However, I did notice a weird issue (although I'm not sure whether this is relevant to the current issues or not) - I can move the gear lever out of park and into any gear without depressing the brake pedal (regardless of whether key-in or out, or key position) . The car does detect when the selector is being moved out of park with the ignition on by making that beeping noise. Again - not sure whether this is at all relevant to the P0715 and the ECM communications issue.
Here are some photos off of the IDS screen (taken after Mongoose was disconnected which is why the battery indic shows red).
It seems that the P0715 is the main issue here. I may have jumped the gun by posting without going through with clearing the rest of the code so that one's on me, ergo I still cannot understand the ECM error.
Lastly, I must reiterate that the P0715 comes back instantaneously: even when I clear the code, cycling the ignition to on simply to complete the code erase procedure, brings the fault back up. So perhaps we may be right in inferring that this is an electrical issue and not a mechanical one?