Colormunki Display also asks to adjust for target Brightness, mine is at 100cd/m2. I have a selection for Native white point. see screengrab below.
I noticed you have ADC selected in Colormunki prefs so I'm wondering if your Dell uses internal hardware LUTs, but then the vcgt tags should've shown a flat line. Just for testing I'ld turn ADC off and that ambient stuff off. Keep it simple.
Also have you checked to see if Windows is loading the video card LUTs upon startup. There's been issues in the past on this but I'm not familiar with current Windows or Mac OS in this regard.
The native white point selection in Colormunki is useless if you can't access the OSD RGB gains. Or you could choose native and accept the look of your current neutral white point. You should just do it this way just to check if it fixes the off color step wedge.
I tried changing the PS working space to the monitor's ICC profile (as shown below), but nowhere does PS show white point XY numbers. I don't follow.
Sorry for not being specific. Once you have your Dell profile as RGB working space click the menu again, hold and scroll up to CustomRGB... A dialog box will show up indicating white point XY numbers as measured by the Colormunki colormeter and written into the icc profile.
To rule out adaptation and surrounding light influences check there's no color in the current native state of your Dell by viewing a white screen filling the entire screen at night with the lights off. If you can't see any color tint, then your safe to set it to native in Colormunki. The colormeter will see native white as one specific tint that you will never see but the profling software will build RGB video card LUT curves and tweak each one to scale that neutral white tint throughout the rest of the gray tones all the way to black.
Right now I only have two control buttons on the Dell 3007: one for brightness control and the other to power on/off. Should I install this OSD software here and manually adjust the whiteness of the display to match what my eye thinks neutral white looks like? And if I do that, the rainbow step wedge problem will be resolved? I don't see how.
You don't need to see how. Just remake the icc profile with target brightness, native white point, version 2 profile, and turn everything else off - ADC, ambient and whatever else. Don't even mess with the Dell software. It's a process of elimination so start out with the least options.
If that doesn't work reduce Brightness to read at 100cd/m2.
If that doesn't work then install and use the Dell OSD software and try that route.
If that doesn't work, use the Colormunki to profile another display using the simple native settings.
If that doesn't work, call Xrite support.