There really is no need to include a shaper in a P3-DCI to P3 D65 ‘sim’ LUT like this one. It is not changing the gamma at all (2.6 in and out). It is when doing significant transfer function change that shaper LUTs become important. You can think of this as more like a calibration LUT. Where a calibration LUT modifies a signal slightly to compensate for the differences between how a display should respond and how it actually does, this LUT modifies the signal to compensate for the differences between a P3 DCI display and the virtual P3 D65 display you want it to behave like. But unlike a calibration LUT, you do want to bake it into the rendered file.
I will reply to your off list email if you want more help with shaper LUTs in general.