'Plinth' is really only used for the base of a statue or pillar, and certainly doesn't have this meaning in everyday English. I don't think your target reader would know what you were talking about. 'Arcade' is a row of shops and may include shop/office type premises, such as estate agencies, banks, etc, often but not necessarily under some sort of cover (often the projecting floor of the next storey up). 'Mall' usually is covered, but might suit your text.