It's good that you are making some progress with your 621.
If you want to get a good steuerpimpel, member Don aka mrow2 makes them as well as selling small containers of alvania grease suitable for cams, and supplying a helpful service guide with all purchases. These tend to work better and should last for years. Don's a good guy and very helpful too if you have any questions, plus based in the US.
The small metal spring for single and repeat play often comes loose in transport if the item has been jarred at some point, but you were smart to hold onto it and find where it goes.
It's unfortunate the hardware holding the electrical box was missing, but as you mentioned perhaps you can find a local substitute at the hardware store.
twice the fun