Diode in series..?
You probably want to avoid series diodes, and shunt shottky diodes, because of voltage drop, and leakage current, respectively. Typical is to use zener/TVS or clamp diode (PN junction, e.g. BAV99) to limit voltage without much leakage.
Mind that zeners aren't available in sharp, accurate voltages; leakage goes up sharply at say 80 or 90% of rated breakdown, and low voltage ratings (under 6V) have terrible leakage period, so much so that they're awful even for voltage regulation purposes.
What's up with the switched common ground? Is that to reduce current consumption? Current still flows between taps though...
Tim