Is it allowed to supply the ADG5413 with 5V when all switches only have to remain open (high impedance)?
During this device state the inputs off the ADG5413 will be 2.5V +- 0.3V
When the ADG5413 is switching, it's supply will be >=9V (Minimal listed single supply voltage).
The 5V supply is required during the state where the ADG5413 only has to remain high impedance due to single fault safety of a medical device.
A alternative is using the ADG5413F because quoting from the datasheet:
The ADG5413F is considerable more expensive though. Since I need 12 of these muxes per device, this would add quite a lot to the cost price. Also the over voltage protection of the ADG5413F is not required.
Unfortunately 5V supply is outside the operational range for the ADG5413 and therefore the switch positions cannot be guaranteed.
The best way to provide power off protection would be to use the ADG5413F as you mentioned.