FlightPathOBN:
the further we get into these boxes, the more difficult it seems to become, and these issues certainly makes automating CDA, idle descent, TOD, and trajectory management difficult, if not near impossible to achieve...
(and they wonder why a public RNP procedure is so difficult to design)
Those sound like issues with design of special RNP AR IAPs for a particular operator.
The only deference the FAA has made to avionics in the design of public RNP AR IAPs (and we have a whole lot of them now) is not less than 50 seconds of flight between the FROP and the DA to accommodate the AB, and that is only if the missed approach is an RNP missed approach (less than RNP 1.0). Otherwise it is not less than 15 seconds. This is to accommodate transition to GA mode. The only other accommodation, and it is very broad, is that an IRU is not required for RNP AR IAPs of not less than 0.30, and without missed approaches of less than RNP 1.0.
It's up to the airframe OEMs to address the multitude of specific avionics issues in order to obtain RNP approach level approval.
Difficult to obtain that approval and for the operator to validate the RNP AR database? Yes.
Difficult to design public RNP AR IAPs? No, at least not in the United States. The only major impediment to public design is ATC and impossible terrain at a very few locations.