[The Saucepan Man]

Originally Posted by Nilpaurion Felagund

And the other choice he was pondering while in Lothlórien--going to Minas Tirith--was made irrelevant for the moment.

Boromir does of course entreat Aragorn to go to Minas Tirith to save his people and Aragorn promises him that it shall not fall. Given his 'destiny' Aragorn is, in a way, already obligated to defend Minas Tirith, but this promise to Boromir on his 'death-bed' is a weighty one to make.

Originally Posted by HerenIstarion

One can't help wondering, what exactly Elrond knew at their setting out?

I think that Elrond was just exercising good common sense:

For you do not yet know the strength of your hearts, and you cannot foresee what each may meet upon the road.

At the outset, no one could know for sure what would befall the Fellowship. A good degree of 'flexibility' was required to cater for the precisely the kind of eventuality that occurs in this Chapter. The only one sworn to follow Frodo to the end was Sam, and he would have done his utmost to stick with Frodo, vow or no vow.