... I would like any initiated NEW PM to be assigned a number which allows the /r command to work in multiples... consider this exchange below:

Sent by NavyChief
/pm joeflagger Hello Joe are you trying to recover your backpack stuff?
PrivateMessage joeflagger assigned as player 1
/pm partialkilla Hey Killa are you on your way to the dome
PrivateMessage partialKilla assigned as player 2

What joeflagger & partialKilla sees:
Private Message from NavyChief: Hello Joe are you trying to recover your backpack stuff? (/r1 to reply)
Private Message from NavyChief: Hey Killa are you on your way to the dome (/r2 to reply)

SO now the exchange between NavyChief and partialKilla only needs /r2 <message> for both players

Maybe the number will reset after no PMs for X minutes (configurable in the json)

It is also possible that the number stays until the initiator logs off or closes it out
/pm 1 close

Someone else might se a better way to do this but I have replied to the wrong PM before and it did not go well.