Hi Sean, my apologies for the delay. Mercedes has a very strict policy regarding theft relevant parts. Any part that has the vin number coded into the part must be purchased and installed from an authorized dealer. There are exceptions to the rule if a shop files the necessary paperwork with Mercedes to order and install theft relevant parts.
Yes a faulty gearshift mechanism can cause the transmission to default to limp home mode. Another possibility is the electro-hydraulic control unit inside the transmission. If the shop is pulling fault codes relevant to the gearshift mechanism replacing the gearshift should solve this issue.
Let me know if you have any questions.