MK2 1.6 TDCI electrical issues

Hey guys! 

Long time reader, first time poster but unfortunately out of ideas so gotta ask the public!

Mk2.5 2008 Focus with the PSA engine recently lost all power while on a motorway around 70mph (engine cuts out, battery + Oil light on as engine not running). After a restart seems to work fine until this happens again. Soon after the issue started occurring at lesser speeds. No codes stored so first thinking was fuel starvation or similar; noticed the injectors have been leaking too so tightened those up until the seals arrive - nuts were looser than anything. Didn't solve the issue and was still cutting out.

Few days later an engine malfunction message appeared on turning the car on although still seemed to start and run albeit in limp mode (didn't drive enough to check - straight home to the driveway)

After these issues invested in a EML cable and Forscan only to find a whole plethora of error codes - Crank sensor, cam sensor, DPF, Fuel rail pressure, EGR - you name it! after clearing these and having another run the codes that do remain are mostly U1900 communications codes. these do however change between different modules indicating an electrical issue so that's where i started!

Checked all earths on the front fender/member and 2 wires snapped in my hands so stripped and recrimped, cleaned the rest and retested - car no longer starts but Engine malfunction message keeps coming up. at this point noticed also that the ambient temp sensor is reporting -60˚ (rather cold right?). Took apart the Front junction box to clean up any bad connections - all appeared clean! Wiring seemed quite damp in the area by the ABS pump under the battery - took out battery jiggled all wiring - Car started and ran! read the codes while at it - IAT and MAF sensors but they were disconnected at the time so expected, and u1900 to the IC. Now this Instrument Cluster error seems to be the reason as many people have complained about this being the issue although it still didn't quite fit the bill - the LED for immobiliser isn't flashing with any error codes and behaves as expected.

Ripped apart the IC to check the wiring behind and the connector pins - all looking as expected - put it all back together - no start again - some quiet relay clicks around the Engine malfunction message and thats it (although the IC does appear to slightly misbehave in a way that some dials only activate after the Malfunction message - fuel for example or when it did run for few times - the RPM).

Started stripping the loom apart to check for broken wires - nothing broken visually but yet to find a non-colourblind assistant to go through all wires one by one to check for continuity against the schematics kindly provided by this site. Weirdly enough - when moving the loom near the bulkhead hole the car starts and at one point the temp sensor went back to normal as well (15˚)

At one point all started working well and only the u1900 code kept appearing so i started putting everything back together thinking that cleaning the last earth inside the front wing would have done the job! once all reassembled went to crank over and nothing - dash lights up, temp gauge at -60˚ and red snowflake, Immo LED stays on for about 3 sec then goes off and cracking the car results in absolutely nothing...

At a total loss right now - is it the PCM? Wiring? the temp sensors makes me think it could well be GEM but that shouldn't have affect on starting, right?

Currently reduced to a bus service for school runs and highly inconveniencing  Ideas would be most welcomed thank you!