Actually, I've found that power failures (and a couple of inadvertent power-offs by yours truly!) have caused absolutely no damage to my Linux system. The file system seems more robust than Windows' system. When I was using Windows, I had real problems a couple of times after power failures and other "unclean" shutdowns, but never yet with Linux. Maybe I'm just lucky.
Anyway, I now have a battery-backup unit that gracefully shuts the computer down normally if the power fails. One of the best purchases I ever made. Exactly a week after I installed it, a storm knocked out power but the computer shut down just as if I'd logged out and given the poweroff command.
But a backup power source does no good if the computer switch shuts the machine off instantly.
I don't know about the Dell machines--I would have thought they had a pretty sophisticated BIOS setup, given their excellent reputation... Are you certain there are no sub-menus under the 20 or so commands? That's how mine works.
I wonder if you couldn't obtain the operating manual from the Dell website and see if there's an option for the switch feature you want. Like you, I've always preferred that the power switch should put the computer in standby rather than power it off with one brief push, just for an extra margin of safety.
If I had a child that might want to investigate the power switch, and I couldn't change the BIOS setting, I think I'd fashion some kind of cover for it, superglue it in place, and shut off power when needed by pulling the plug (recommended by many experts anyway as the only truly effective protection against lightning damage).
Hope you can get what you want done!