Yes it is the upper right passenger side carpet retainer (the sloped section pretty much under the dashboard), mine just pulled off and I didn't know it was there, no reference was ever made to it, only the screw-on retainer for the black foam under the carpet. The stud for it is attached to the body under the carpet, and sticks up through the carpet when properly in place.
No, no, no. It is a right threaded Muffler Bearing Retainer Ring and your lucky to have found it as it is on the unobtanium list for 2017. When replacing, please don't forget to reset your maintenance meter. These things are good only for about a thousand miles or so and Mercedes parts suppliers have cornered the market on them. They are no longer covered under warranty unless you can show evidence of non-tampering.
Of course, I could be mistaken. I am not a Smart Car mechanic, but I did sleep in a Holiday Inn last night.