Did Della take out the rocket, knowing it wasn't meant for solo trips, because she had postpartum depression after the triplets were laid? Or was it an attempt to prove that she could still be an adventurer and explorer even though she was a mother now? And either way, if something bad happened to the triplets' father, did that have something to do with her impulsive decision?
Is Scrooge's greed straightforward? Or is it because he wants to save up enough money to be able to continue funding the space program to find Della? The latter seems most likely.
"Scrooge, I took the Spear of Selene. I'm sorry." Now we know the full story, and why Donald and Scrooge didn't speak for ten years.
"Crazy old man! All you care about is the next adventure," Donald chastised. Huey in "The Impossible Summit of Mt. Neverrest" called out Scrooge for putting his need to reach the summit over common sense. When Mrs. Beakley comes along on this adventure, she calls out Scrooge for seemingly doing this on every adventure.
There's also the fact that Scrooge seemed to have taken Della's side in the argument Donald and Della had about going adventuring (to the unexplored reaches of space, no less) when now there were little ones on the way. Donald was worried about the boys, Della was confident nothing bad would happen, and Scrooge seemingly decided that adventure was more important than the safety of a newly enlarged family.
Donald dropping the nephews off at Scrooge to watch for the day and asking if he can "do it without losing them" carries a lot more weight now, especially Scrooge's reaction after being asked this.
Donald's "This is the Spear of Selene all over again!" is harsher knowing the circumstances. Donald isn't angry at Scrooge for taking Della someplace dangerous (as the audience had been led to believe), but for enabling her recklessness by giving her a space ship behind Donald's back, the way Scrooge is enabling the kids' (especiallyDewey's) recklessness by taking them to Atlantis behind Donald's back, instead of making them stay home where it's safe as Donald tried to do. Considering Donald making Scrooge The Scapegoat over Della's decisions and Scrooge secretly blaming himself deeply for her disappearance...
Scrooge's comment about hiring people to be his family so they'll have to listen to him after Dewey ran off for the umpteenth time; the implication that he tried to get Della to turn around and when she refused he resigned to guiding her through the space storm.
"From the Confidential Case Files of Agent 22," when Scrooge tells Webby it's strange they never spent time together before, Webby innocently repeats that her granny told her not to bother him since he was busy with his business and grumbling... And Scrooge briefly looks away guiltily. With The Reveal that he was searching for Della (which Word of God reveals he was at for nearly ten years), whom he blames himself for losing, it's a double-whammy: not only does Scrooge feel guilty for ignoring Webby all these years, but of his fruitless search for another young female duck ward he had failed.
"The Last Adventure!" changes the entire episode when it's revealed that Bradford told Della about the Spear of Selene in an attempt for her to take it for a joyride and hope that Scrooge would be broken from adventuring. Scrooge telling Webby that she's not family also hits a lot harder given that she's actually his daughter.
For the matter mentioned in the spoiler, it also counts as a Rewatch Bonus. Pay attention to the Buzzards when they are shown in the Troubled Backstory Flashback into forcing Scrooge to give up his search for Della. Bradford is shown can be seen in the background with a devious smirk. Him confessing that he told Della about the Spear of Selene shows that he knew exactly what he was doing and that everything was going according to plan. This also means that Scrooge and Donald got mad at each other and stopped speaking for no reason at all.
And speaking of "The Last Adventure", Scrooge shuts Webby up by stating, "This is a family matter and you are not family". Technically, she's his daughter so yes, she is family.
I Knew It!: Quite a few fans guessed that Della's disappearance would be inspired by the Dutch comic story where she disappeared in a rocket. Related to this, many fans also guessed that the "Spear of Selene" is the name of her spaceship.
Older Than They Think: Some people were surprised that it showed that the triplets were hatched from eggs and believe that this is the first time any Disney media has acknowledged this, even other Disney Duck cartoons and comics have occasionally referenced this too; the original cartoon even had a nod to this in the episode "Sweet Duck of Youth."
Even though he makes a silent cameo being shown as one of the employees walking out on Scrooge, Duckworth says nothing to him.
Since there is only a couple of minutes left in the episode, we don't see Donald dealing with the consequences of the boys finding out about Della.
The Woobie: All Scrooge wanted to do was give his niece a nice surprise to celebrate the birth of her kids. Instead, he wound up with his niece disappearing, a large portion of his fortune wasted trying to find her, and his nephew cutting off all contact with him and taking his great-nephews with him. Years later, after partially mending fences with his family, his great-nephews discover what happened to their mother and turn on him just like their uncle did.
Donald becomes this as well when we learn the circumstances behind his sister's disappearance. As far as he's concerned, his nephews had to grow up without knowing their mother all because Scrooge chose to side with Della by building the Spear of Selene instead of telling her to take some time off to focus on her offspring.
