1) RT doesn't add anything automatically, but like all raw converters it allows you to add contrast via a curve, but unlike most raw converters you can choose which tone operator to use, "Standard = RGB", "Film-like = ACR", "Perceptual = similar to DCamProf", and a couple of more. With RT you can choose which elements of the DCP to activate, for example you may skip activating the looktable and curve, which for a typical DCamProf profile would leave only the colorimetric aspect. Anyway you don't need to worry, per default RT behaves as you would expect, it's just that you have so much more control, a bit too much control to be that software that everyone wants to use, but it's nice when you're a control freak
2) Uhmm... there's been some changes back and forth on when the NTRO gets activated or not, and I'm really focused on the GUI version so I'm not really sure without testing what happens with a particular parameter combination in the latest version. What
should happen though is that as soon as you have a parameter that requires the NTRO to be activated it will. For example if you have linear curve and no gamut compression there will be no NTRO as it would make no difference. If you activate gamut compression, the NTRO will be activate even if you have linear curve as it depends on that, but really the basis of using a tone reproduction operator is that there is a curve other than linear. If you use a linear curve I've assumed that the user is making a profile for reproduction, and then you just want a colorimetric profile.
I'm not too familiar with Iridient so I don't know what it does if the profile lacks an embedded curve, but it would be easy to find out by testing, or just ask Brian.
3) The best is if your target reference file contains spectra, then XYZ values can be recalculated from scratch for any illuminant you choose. If you measure your own targets with a spectrometer you should be able to make one with spectra. If you're using a file provided with DCamProf they all have spectra. If you've got the reference file from the manufacturer it's likely not containing any spectra, and then you need to know what illuminant that was used when the XYZ values in the file was calculated. It's nearly always D50 or D65 with 2 degree 1931 standard observer. Unfortunately there is no standard field to provide information of which illuminant that was actually used, but often it's stated in the description field or some non-standard field, so look at the text file and search for D50 or D65 and see what you find. For example you can find something like this:
[DESCRIPTOR "L* a* b* Batch average data (light D50, viewing angle 2)"] and then you see that D50 with the standard 2 degree 1931 observer has been used.
Exif tag 4? What's that? Flash can generally be approximated to D50. An exact match is not really that important. When the calibration illuminant is said to be different from D50 there will be chromatic adaptation transform applied, the reason to simulate the slight appearance differences of colors under different light, making reds a bit brighter for StdA for example. In theory the color temperature estimation of the DCP becomes better with an exact calibration illuminant, but that's only in theory, and for single illuminant profiles it has no effect, and if you use a raw converter like Lightroom that takes custom wb from the temperature estimate rather than storing the true RGB multipliers you'd probably want to embed the original ACR color matrices anyway. I don't know how Iridient developer does that aspect.