The parable shows a few things that IMO are more important than the things you mention:
Disputes are not necessarily resolved when the first agreement is reached. If the border was important enough to fight over, it should be important enough to fight again when one side tries to move it (or when it passes some threshold. This is a problem in the real world, too—https://en.wikipedia.org/wiki/Border_irregularities_of_the_United_States has a number of examples of rivers that move, and the borders sometimes move and sometimes don’t, but either choice requires discussion and agreement.
Schelling points/lines are useful for identifying candidates for agreement, but they’re not enforcement mechanisms. There must be some underlying reality that the agreement is about.
The parable shows a few things that IMO are more important than the things you mention:
Disputes are not necessarily resolved when the first agreement is reached. If the border was important enough to fight over, it should be important enough to fight again when one side tries to move it (or when it passes some threshold. This is a problem in the real world, too—https://en.wikipedia.org/wiki/Border_irregularities_of_the_United_States has a number of examples of rivers that move, and the borders sometimes move and sometimes don’t, but either choice requires discussion and agreement.
Schelling points/lines are useful for identifying candidates for agreement, but they’re not enforcement mechanisms. There must be some underlying reality that the agreement is about.