OK, to summarise, you have:
Replaced the ABS/DSC pump
Replaced the ABS/DSC ECU with an ECU that has the EXACT same part number and coded it to match your car (or refitted the original unit)
Replaced the front left and front right ABS Speed sensors
Replaced the ABS sensor on the master cylinder (not sure about this one - haven't seen it myself).
Checked ECU plug/socket for moisture/corrosion.
I'm really not sure what's left. If you are not getting codes to say the ECU is not communicating with other modules (over CanBus) then it doesn't sound like a wiring fault, but that may not be prove-able without load testing the wiring to recreate the fault.
The warmer weather appears to have cleared the fault, so that points back at a wiring fault, e.g. moisture in the circuit. It will be interesting to see if the fault reappears in damp/wet weather again.
Sorry I can't be any more help, I'm not a mechanic or auto electrician, but yours is the first 5DF0/5DF1 fault I've read about that wasn't the pump or ECU.
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