I have year 17 Ramesses 11 = year 8/9 wHm mswt. So the instructions of Ramesses 11 to KSOK Panehsy, royal scribe of the army and overseer of granaries, would overlap the time when HPA Piankhy used the title of KSOK. As I pointed out: every chronological model has problems. This overlap in my chronology is one reason why I see Piankhy and Panehsy being at odds with each other and with Ramesses 11 favouring the latter. The situation may reflect similar conditions, some years earlier, when KSOK Panehsy appears to have been called on by Ramesses 11 to march north and remove (for the second time) HPA Amenhotep (at least in my model).
On your double dates - I see you do not just get it - you are too fixed on civil years as a reference point. Look, in regnal year terms, 1/I Akhet is just another day in the regnal year. It may have a special name but is not chronologically significant in terms of time measurement. Imagine if 25 March were new year's day and not 1 January. That is a new year began of 25 March (rather than on 1st January) and the following 1 January was just another day in that same year. Given your conceptualisation you may find this idea rather stupid but bear in mind that such a model was the official calendar of England until it switched from the Julian to the Gregorian calendar in 1752. And that was only in England, some of it's colonies continued to use the old style calendar for some considerable time - i.e. the Americas where George Washington was born on 11 February 1731 (Julian, 25 March start) whilst in England it was 22 February 1732 (Gregorian, 1 January start).
As for year 1 = year 19 - I have said this several times before - regnal years, for kings who reigned consecutively after each other, could only be referenced to an era, not one another's regnal year. The only separate era known (covering the period of Egypt's pre-Greek pharaonic history) was the wHm mswt era, in operation just at the time these double dates occur. If year 1 wHm mswt = year 9 [Ramesses 11] were the start of this series then the logical thing to conclude about year 1 = year 19 is that it comes at the end of this series. So I have suggested: year 1 [HPA/King Herihor] = year 19 [wHm mswt].