I lived in Spain in the 1970s stationed there in the military. We lived off base and I can understand the moisture problems with masonry; we had some in the apartment building we lived in.
The main thing is air circulation. I would get a medium sized, say 600 mm, box fan and just have it running in the garage. As long as the air is moving things will stay dry or dryer. The chimney will help a lot in allowing humidity to escape. Where I live now we get periods of freezing weather followed by a warm front that moves in. When everything in my garage is freezing cold when the warm air moves in the stuff will sweat. Having a fan moving air keeps stuff dry and sweat free. Keeping bikes clean and polished greatly reduces corrosion.

You could check with city or municipal codes on walls, setback, easement rules that it seems the neighbor may have violated.