Codes 51 and 31 are related.
51- oxygen sensor
31 - mixture regulation auto adaptation

So I think the lambda sensor is gone...

That screw does not count for the DFW 148bhp engine. The engine management does not take it into account.
It works as CO pot only in non cat versions.
I know as I have one of each...

I have one of each, a D6C from 87 and a DFW from 89. The DFW idles a bit high (950rpm) but very steady, and the D6C idle is a little bit lumpy but only 850rpm. Yours is definitely lumpier than anything I have come across. Possible faults: - AFM track worn, false air ingress after the AFM, faulty thr...

ET 15 for the original rims and 22 for the 205 1.9Gti. They are the same width , 6inches.
Xantia 15in alloys are ET 15