Yoda says that line in response to Obi Wan calling Luke their last hope. Which technically he isn't, as long as there's another person alive who could do the same job. Crash training Luke to fight Vader always seemed like a long shot anyway, so I think they're taking Han's "Never tell me the odds" stance on the whole situation.
Also, your point about "The Other":
Firstly, it's:
Obi- Wan: That boy is our last hope.
Yoda: No. There is another.
Yoda's saying there is another hope, in response to Obi-Wan's statement. Him phrasing it as "another" is perfectly suited to the context. He's not saying "don't worry, we've still got X", just that Luke is not their last hope.
In the scene on Dagobah where Yoda and Luke are discussing seeing the future, it's implied that not only has Yoda seen what Luke saw, but he saw more than Luke did. While Yoda says "always in motion is the future", he still tells Luke that he could help them, though it would "destroy all for which they have fought, and suffered".
And as for the underlined, let's not forget that it wasn't Luke that saved Leia and the others, it was Lando, with a little help from R2-D2, while Vader was distracted fighting Luke. So even if Luke was killed fighting Vader, Lando could still have gotten Leia and the others to safety.
So, it's really a question of how much Yoda saw. For all we know, Yoda saw the whole thing play out, but didn't want Luke to go because it would blunt his and Obi'Wan's plan.
And the fact that Luke only indirectly helps in Leia's rescue, in a way that won't change if he'd died in that confrontation, means that your last point doesn't actually work. Provided Leia survives in the event of Luke's death, and it seems fairly clear that she would, there's nothing stopping her being the one Yoda's referring to.