Sounds to me like you need a good MB shop. Go to MB Shops in this forum and search Washington. Has to be someone. I sold my '91 190E to my grandaughter and she ultimately needed a new cat to pass CA smog. A local indie handled everything including the smog test and it was about $600 (which grampa paid)
I always let my mechanic handle the smog tests. They always pass. Sounds like you have other problems that a good indie could have diagnosed in the first place.
Anziani
'93 300CE 174K
'95 E420 117K