How many miles on this car?
Was the transmission fluid & filter ever renewed?
Ask an M-B dealer to run a VMI (vehicle master inquiry) on your vehicle... look for any transmission work performed under warranty or as good will.
A certain amount of metallic debris in the fluid pan is normal... the question is what type is present... M-B 722.6 service manuals present photos of acceptable and unacceptable deposits. Have the shop show you the fluid pan... stick your finger in the deposits and see what it feels like. Are the deposits gray or brass in color? Do the deposits feel chunky or smooth? Are the deposits ultrafine... or beach sand size or larger?
And what about all the fault codes? Do you trust these guys... is the vehicle truly displaying 23 ETC.6 fault codes? You really need to know what codes are displayed if you are going to make an educated service choice.
I wouldn't necessarily trust the independent shop to be experts on this type of transmission... and if it turns out you need another tranny, I'd purchase a rebuilt unit from M-B before having the exisiting unit rebuilt locally. The factory rebuilt will have a better warranty (2 years/24K miles) and include all the factory upgrades. But you may need to install a newer control unit with the factory rebuilt.
Request the independent shop query the transmission control unit in your vehicle and provide you with the M-B part number. Then call M-B parts, supply the part number of your existing control unit and ask if it would need to be upgraded if you installed a factory rebuilt transmission.
On top of all this, you still may have a bad gear recognition switch that is keeping the vehicle in limp-home mode.
Shop won't provide you with the fault codes? Going to spring for a rebuilt tranny anyway, if all else fails? Want to throw caution to the wind and have a go at this yourself? Want to spend $300 (or less) on parts, plus towing, with no guarantee it will work? Then pull the vehicle from the shop as is... don't have them refill the tranny... the fluid is approx. $10/qt.
At home, remove the fluid pan and drop the valve body.. held in place with less than a dozen torx head bolts. On the top of the valve body is the conductor plate. Remove the plate and transfer the existing solenoids to the new plate (about $125). Install the completed conductor plate to the valve body; reinstall the valve body. Install a new fluid filter and a fluid pan magnet. Renew the wire harness adapter plug. Renew the transmission fluid, using the M-B fluid specific for the 722.6 tranny. A transmission control unit upgrade is not required for a conductor plate renewal. Treat the entire service procedure as though it were open heart surgery - use sterile technique... no dirt anywhere. If you can't adhere to that requirement, then don't do this. Before you start, spend $20 and a day at www.startekinfo.com.
Familiarize yourself with the 722.6 transmission.
Still only runs in limp-home?
Then replace the gear recognition switch... its less than $100 and they haven't had a stellar reliability record. Replacement is a bit tedious but not technically difficult.
This is really all you could do at home. Its a gamble. Your call.