"My name is Ozymandias, king of kings: / Look on my works, ye Mighty, and despair!"
Never let the future disturb you. You will meet it, if you have to, with the same weapons of reason which today arm you against the present.
does one have to have a city/settlement on one of the two specific corners of the hex where the harbor is...?
Yes, the city/settlement has to be on one of the two coastal vertices specifically bordering the harbor.