Solved! Here’s my findings, just in case someone else runs into this issue.
What I was doing initially: Let’s calibrate this baby, using the setup above, shining light from my hacked enlarger that I use for DSLR scanning (full description can be found here: Let's see your DSLR film scanning setup! - #179 by ArnoG) onto a X-rite Colorchecker Passport as seen above, using the 18% graycard in the Colorchecker Passport for the (reflective) white balance, and the X-rite LR plugin to create a custom camera profile. I get:
Measuring the fourth grey pad from the left on the bottom I should get about 50% grey according to X-rite and I measure exactly that, using the custom white balance that results in a color temperature of 4,300 K and a “tint” of +36 in LR. My reflective target seems spot-on, so I presume that my system is calibrated using the 4,300 K, +36 tint, and the custom camera profile hence generated. Scanning my Velvia 100 slide with this profile and white balance, however, results in the off-color LH picture above and the middle picture in:
Based on Harry’s finding (thanks!) that taking a white balance off the paper that the girl is holding gives something closer to what it should be, I suspect no longer a profile error, but indeed white balance, and since I use a Solux daylight bulb, indeed switching the WB to daylight (5,500 K and tint +10 in LR) improves things. My Solux bulb (CRI 99+) says 4,700 K on the box:
So I plug in 4,700 K, tint 0 into Nikon’s NX Studio using the Nikon proprietary “camera neutral” profile and get the picture on the right, which seems very close to my slide.
Using LR, with WB = 4,700 K, tint 0, and my reflective target custom profile things seem too reddish and bold, but using Adobe’s version of Nikon’s camera neutral profile, or Adobe’s own color neutral profile for the camera, results in the pic on the left with only a slight exposure adjustment (EI +0.75), which is very similar to what Nikon’s NX Studio produces (right picture), and similar to what is on my slide. Which profile is used (Negativelab 2.3 neutral, Adobe Color neutral, or Adobe’s neutral version of the Nikon camera profile) only has a minor effect on the color balance.
Lesson learned: A reflective target calibration cannot be used for transparent media scanning, which is what Mark was stating (thanks!).
Overall, apart from creating a custom calibration using a transparent IT8 target which could improve things further (I might do at some point), I’m pretty close already, and at least the colors are no longer horridly off.
I’m still puzzled as to why a reflective target calibration cannot be used for transparent media, but alas, my setup is producing proper results now, pending a further custom calibration using a transparent target for fine-tuning.
Thanks all for the useful inputs and help.
Arno