As far as I'm aware the formation of the Ulster Volunteers by Carson wasn't a move to force partition rather it was to be used as a force to resist imposed home rule. At what point did unionist begin to suggest it as an alternative?
Under the Anglo-Irish treaty the Boundary Commission was established which the Free State government were told would involve a redrawn border involving the transfer of large areas of land south. The commission only suggested small transfers and ultimately was rejected by all sides.
But from reading on the issue it seems that many of those in government and civic society in the south saw partition as a temporary inconvenience with unification an incoming eventuality. At what point did this viewpoint change?
I can't speak to the history of partitioning (or at least not in the depth that would be required for a suitable answer in this sub). But as to the question of when the Irish government accepted that the split was permanent, arguably that didn't happen until the signing of the Good Friday Agreement of 1998, in which the Irish Constitution was amended to remove claims to sovereignty over the North.
Unfortunately, that falls foul of this sub's 20 year rule.