Actually, you don't have to remove the steering wheel on the W123. Take the boot off the switch, then remove the two screws holding the switch in, two wires for the horn, and it comes right out by fishing the wire harness through the steering column.

As for the "turn signal shaft", are you talking about the steering shaft? If I am not mistaken, there is simply a steel block welded to the shaft that the switch contacts to trip it and return to center. Unless this block fell off, any non-centering problems are likely in the switch itself.

As for the windshield, you may want to look into new instead of used. Some new windshields are actually very reasonable in cost. Here is one site you can check that will give you an immediate quote:

https://www.safelite.com/sslPages/service.jsp?CLICKED=QUOTE