2010 Mercedes ML350 Bluetec Diesel

Hi all;
I have a 2010 Mercedes ML350 Bluetec Diesel which I acquired about 6
months ago and so love it. My engine warning light came on about a month
ago and I took it to a mechanic I've just started seeing for regular
maintenance. They have foreign cars for sale and also service them. They
scanned the vehicle and said the service code that came up was for a
malfunctioning exhaust system EGR valve. They inspected it, tested and
couldn't find any visual fault with it. They reset the system and test drove it
afterwards but could find no drivability concerns, and the light remained off.
They recommended i drive it again to see if the light came on again which
they thought would indicate an internal fault with the EGR valve. After a few
days of driving, the engine warning light came on again. I took it back to the
mechanic, they inspected it again with the same result but determined that
the EGR valve must have internal malfunction and that it should be replace
for $1200 - $2000 depending on part cost. They scheduled the repair and
purchased the part for $800. I have now been driving the vehicle for 2 weeks
and the light has not come back on. Can someone give some advice as to
whether the EGR valve should actually need replacing if the engine light is
remaining off for this long? If I was to take the vehicle to another repair shop
would they recommend the same service? Or could this be another simple
problem? I have some concerns about the judgement the current service
mechanics are making... that I should spend this amount of money when
there is no concrete evidence that the EGR valve is faulty.... much
appreciated