I'm not a tech, so take this advice for what it is worth, (not much).
Yes, the hole you are refering to is the mixture adjust point.
There may be more to adjust that I don't know about yet. To adjust,
you need a special, long allen key wrench. I don't have one so
I can't tell you how long. I think that the Merecedes CD shows the
tool with it's part number and possibly it's length.
Don't change the setting much; you don't want to make it worse.
As for running rich, as far as I know, you can't hurt much.
You likely work the catalytic converter more and have higher