Lots of error codes. Need help diagnosing root cause.

[tcpaulh]

Volvo S40 SE
RE05 XAH
1.8 Petrol
Engine designation according to manual B4184S11
Vin YV1MS214252102870


Bought car a couple of years ago and started getting "reduced engine performance" on dash readout. Garage determined throttle position sensor faulty and replaced pedal+sensor

Car uses 1/2 a litre of oil per tank of petrol. Drives well 35mpg.

Last week I was on a 200 mile run and stopped off for a break. Reduced engine performance dash warning had been showing for months but I couldnt afford to have it looked at or was ignoring the problem...
Car refused to start. No life from engine and a bunch of dash warnings as attached

Recovery service called out who couldnt start it at first but then after a failed bump start (no engine activity) the car came to rest and after that it started as normal.

Took to garage and they found:

Error Codes
2138 - accelerator pedal position sensor faulty signal
2121 accelerator pedal faulty circuit *
D900 Configuration fault. Software faulty
C168 Intermittent fault. Commmunication brake control module (BCM) signal too low
C400 Control modulle communication ; faulty signal
C167 Intermittent fault. Communication central electronic module (CEM) signal too low.

*Cleared all codes and 2121 returned. Pedal+postion sensor replaced (for the second time)

Paid for work. Started car and "reduced engine performance" on dash. Turned off engine, spoke to mechanic as still at garage. He turned ignition on and error no longer displayed so drove home keeping fingers crossed.

Car parked up for 3 days then today I went to start it and got a bunch of errors on dash readout. Same as before. No life from engine. Lights come on etc as normal.

Questions

Whats wrong?

Have I got any reasonable comeback with the garage?