My Mercedes benz C200 W204 2008 version only cranks and won't start

Hi All, I need some help here. I own a Mercedes benz C200 W204 elegance model. The last I used it was 3 weeks ago....drove 7-8 Km an then let it stand for a whole week as I had to leave town on work. When I got back the following week, I tried starting the car, but the battery had gone down. This is the first time this happened, as I always use the car mostly on the weekends only and it always starts.
After charging the battery, the car only cranked but did not start. The battery guy refereed me to a local mechanic who came with a OBD2 scanner and got the following report - "The output for the fuel pump has an electrical fault or open circuit". He told me that probably rats have bitten off a wire (probably to the fuel pump) and that he would surely fix the problem, but I would need to tow the car to his shady garage and leave it overnight, which I was not comfortable in leaving my car with someone I didn't really know.
So I called another Mech (who I heard was very experienced in these cars) and he came and immediately removed the back seat to check the fuel pump. He remove the connector to the fuel pump and turned on the ignition to the fuel pump with a continuity tester in between the loose end of the connector and the other attached to the fuel pump. The lights came on and he suggested that current / voltage is coming up to the fuel pump but the fuel pump is dead !!! I now need to shell out approximately 20k to change the fuel pump. (called MB and they gave me a quote of 40k)
I am still confused, what if this is not a fuel pump issue ? what if the fuel filter is choked? what if a relay is not functioning right? the other local mech told me very confidently that he would get my car started...like he knew what to do and where...
Appreciate any help or advise in this matter.Thanks.