[QUOTE=Skald the Rhymer]
Frodo leaves as describes; remember, he and Samwise didn’t know about the abduction of the two younger hobbits. A similar debate as in the book takes place as to the the best course of action, and Boromir decides to accompany Aragorn, Legolas, & Gimli to save Merry & Pippin.
[/QUOTE]
I agree that Frodo would leave the same way. Sam already realized that Frodo intended to, and was just waiting for the moment. As soon as Boromir tried to take the ring (the event being left in by the OP), it sets in motion that Frodo runs, and is finally ready to cross; also Sam, expecting this, goes with Frodo.
Also, the orcs are already there, and hunting for hobbit; as soon as they find some, they will try to take them prisoner, kill any non-hobbit available, and then leave with their captives. So, for Boromir to be alive, it must be the case that he is either not with the other two hobbits, or that he defeats them, or is temporarily held at bay while they are taken - in all cases, either with or without the others of the company. In that case, I would think they would all immediately set off after the orcs, and perhaps have caught them sooner. Then, Aragorn and Boromir both wanted to head to Minas Tirith. What then?
Without Merry and Pippin, I don’t think the Ents get roused. Without Aragorn, Gimli, Legolas, and the ents, I don’t think Rohan gets saved, who would turn the tide later at Pelennor.
Aragorn certainly does not, then, go through the Paths of the Dead, defeat the Corsairs, use the Palantir to goad Sauron into an early attack, and gather other Gondorians from South Gondor.
On the other hand, with Aragorn coming to Gondor, Denethor not in grief over two sons deaths, and Boromir there, I think the ‘political’ situation gets sticky, and the people don’t cooperate as well. Minas Tirith falls.
Clearly, Boromir, through his death, saved the entire kingdom! (Said only half, well, three-fourths, in jest.)