Non-latching valves are much more common.
I suggest a simple test to verify your valves work. At your controller, manually connect the power supply wires to the wires for one of the valves briefly, like 1 second. If the water turns on and continues to flow even after power is disconnected, they are latching valves. Apply power again to toggle the valve off. If you get no water flow, connect the wires for a longer period and verify that water is flowing, then disconnect the power wires and verify the water stops. If this works you have non-latching valves. This test will also determine if your power supply is good (able to supply enough power to activate the valves).
The 2.5V higher 26.5V power supply should be OK.
I haven’t heard of any model of RasPi being unreliable. Mine is a few years old model B and it’s been working fine in the garage for 2 years.
When the power goes out everything will turn off, unless you have latching valves. Non-latching valves work simply by turning on when power is applied, and turning off when power is removed. Latching valves turn on, and stay on, when given say 1 second of power – then turn off by say another second of power.
A watchdog would be nice, but mine has been running without lockups for 2 years.