dangerous retaining wall on neighbor's side

What is the name of your state?xxxWhat is the name of your state? California.

I recently purchased my first home in January. A week later, Los Angeles was hit with a series of rainstorms causing landslides. My house sits at the base of a hill and has a retaining wall in the rear holding back the hillside of the neighboring property. January's rains increased the hill's load on the wall and now the retaining wall is coming apart. The city has ordered me to fix it, but my neighbors and I have since had a survey done and discovered the wall is on their property. The wall is very old and no one knows when it was built or by whom. So, who is responsible for fixing the wall?

Also, it turns out that my garage encroaches one foot on the above neighbor's property and three feet onto another neighbor's property. I don't know when it was built but it has been there at least 4 years. No one has asked me to move the garage but I suppose they could. What recourse would I have?

How's this:

Agree to share the cost of rebuilding the wall in exchange for an agreement allowing the encroaching structure to remain and offering them a quit claim so that you cannot later claim that encroached upon land as yours by adverse possession.