You still should be able to get an e bay 4 button flip housing. When you take it apart there will be a circuit board for the remote and a small rectangular plastic piece that contains the RFID chip. Look up a PFC7930 chip.

There is an 8 active key max however each key can have 3 versions but not all versions can be active at the same time.

I'll call the 8 keys A through H, any one or all of these can be active. Key " A " has 3 sub keys that I'll call A1 , A2, A3 however only one of the A keys can be active. Any key can be blocked ( this is reversible ) or deleted ( this is irreversible using regular MB tools however the aftermarket has solutions )

A flat key isn't a valet key. Valet keys are cut slightly different and only work in the door and ignition, they won't for the trunk , glove box and other safe areas.

When I got keys for my 97 SL320 , the dealer said the MB depot had a record of what keys were issued to that car and won't reissue an existing key.

There is a secret handshake for the DAS system. The RFID key chip sends a code to the DAS , DAS looks for it to be valid then sends another code to the ECU to allow fueling. Also in tandem, the DAS sends a new code to the RFID key chip so it is ready for the next cycle. This is a rolling code.

Remotes use a rolling code list, this is why they need to be programmed / synced to the car. The remote and receiver both have a list of codes, the remote sends a code and the receiver looks at the list to see if it is within say 16 lines. This allows for pressing the remote out of range of the car. Each time the remote is pressed a new code is sent.

An even more complicated system ( that you car probably does not have ) is a seed ,challenge , response. In this case the key sends a code to the receiver, the receiver sends a different code back, if that code is what the key expects, it sends another code to the receiver to allow the car to run.