Apple - Mac does not automatically restart when power is re-applied after `shutdown -hu now`

The NVRAM setting to power on when power returns has nothing to do with the shutdown command, so I wouldn't worry about the shutdown options and instead focus on the NVRAM settings for what to do when the machine senses a return of power.

If you don't have an actual UPS - I would get an internet enabled power switch like WeMo or other or just train staff to power cycle the hardware.

The setting to power on automatically is quite reliable in my experience, so you should only need that to recover from 95% or more of the power failure events you have. Also, kiosk machines generally just run apps and open files, so you don't have work that needs to be saved to disk. I would just let the power cut from the Mac and make sure the filesystem is journaled so that the restart is quick.

If you have an actual UPS - use the energy saver control panel to tell the OS to shut down N minutes before the power runs out and you will be well protected from data loss.

Here I have a server with storage attached so I'm having the shut down scheduled 10 minutes before the battery expires. For a kiosk, I would disable all three sliders/check boxes for UPS or get a simple UPS like APS BGE70 that lacks USB port entirely and just gives you some time when people accidentally trip the power or a substation feeder shifts and your outage is a minute or so in length.