"But he soon found his battle-torch extinguished." The sword given to Beowulf by Unferth was a failure. Rather than help him it got him in much more trouble, fighting a beast with no weapons. This might suggest that Unferth wanted Beowulf to die, because he was jealous or wanted revenge for being embarrassed.
"Only Beowulf could wield in battle" Beowulf might've been fated to win this battle. If it had been any other warrior, they would not have been able to lift the sword.
"its blade had melted" Grendel's mother is truly evil and her blood melted an ancient very powerful sword. The sword might've been there just for that battle and after it had been won the sword disappeared.
"marked him from the very start for a happy life" Beowulf is marked for a happy life. But he needs to be careful because this other king had gone crazy and was a horrible king who killed his subjects and was dishonorable. Possibly a foreshadow to what will happen to Beowulf or that god is not happy about the death of Grendel and his mother.
A need for rest came over the brave shield bearing Geat." Beowulf is not a god or invincible. The battle wore him out so he is human just a much more powerful one.