The essential part of what you are describing is that you have a close tie in with a Lab
that can predict what colors are going to result from a recorded negative. The color space is
so different in a subtractive system like film, that some colors will still appear to react more
strongly than desired. We basically left the system as – a ‘close coordination’ with recording vendors
is required. The Rec709 path is marginally safer, but it can be limiting in the look achieved.
The wider gamut of P3 is achievable in part with the right recorder LUTs for conversion.
ADX to ACES is not really an IDT like Alexa, but the principle is similar. In this path, there is an attempt
to ‘unbuild’ the film back to scene relative exposure. An approximation of the average film performance in terms of interlayer cross color effects is used in this process. Going the other way is similar – ACES to ADX – there is an attempt needed to simulate the effects of the film dye interactions so that then a film print preview would get reasonable close results. Closing the loop though between the hypothetical model and the actual output on a recorder is really a job for the house doing the recording that depends on what aim densities are being calibrated to on the recorder.
The inverse path can work as well, although there are sometimes bit-depth resolution issues that can cause banding in an inverse path. The important part here is that it would be better to have made the grading decisions for Rec709 rather than the wider values in a DCDM. The results will be a little easier to get in the smaller gamut.