Your solenoid (rainbird 100-HV) seems to draw much lower holding current than a lot of other sprinkler solenoids. A lot of them I’ve seen draw about 500~600mA inrush and 200~300mA holding current. Since yours only draw less than 100mA holding current, there should be no problem driving 5 valves at the same time.
The ‘ground’ wires on expander cable will bear the return current from all solenoids connected to the expander, and there are 2x ground wires on the cable, so it should be ok for your application. If all 5 open valves are on the expander , then the ground wires will withstand the highest amount (5x) of holding current; however, if only 1 open valve is on the expander, then it only needs to withstand 1x holding current.
From the spec sheet of 100-HV the coil resistance is around 70-85 ohm. Please note that since you are using 12V input power supply, this is going to generate a higher holding current than spec (12/coil resistance = somewhere around 140mA to 170mA). This is ok but not efficient. If you can find a 9V or even 7.5V power supply, that would be more ideal.
For DC-powered OpenSprinkler, please note that the firmware always serializes concurrent running zones (i.e. they will be separated apart by 1 second even if you want to turn them on at the same time), to reduce the impact of inrush current.