Just to be clear your not talking about the relay coil. That is DC. Im a bit confused because you refer to it as an AC coil but the relay is DC and the solenoid on the water valve can not be altered. It is a sealed unit.
Ive looked at many posts and research. The solenoid cannot be opened. I just want to be sure i understand where you want me to place the component to protect the solenoid.
D5 alone would be used to let the negative half of the AC past the LED to prevent the reverse voltage from destroying the LED. But D4 blocks any reverse voltage from getting to the LED, so there's no reverse current for D5 to carry.
The shunt diode protects the LED from any possible excess reverse voltage across it, due to the series diode & circuit capacitance and possible leakage. That's the more critical one for LED protection.
The series diode just halves the dissipation in the series resistor.
With "ideal" components only one should be needed, but there is no such thing in real-parts...
D5 alone would be used to let the negative half of the AC past the LED to prevent the reverse voltage from destroying the LED. But D4 blocks any reverse voltage from getting to the LED, so there's no reverse current for D5 to carry.