The reason Della was screaming out the iconic Moon Theme when she went full Leeroy Jenkins and attempted to jump off the moon's gravity: it's the lullaby she used to sing to her kids. The tune reminds her of them and steels her determination in order to get back to them. Not that it helps her much in that case.
The presence of the Oxy-chew seems to be an Ass Pull as to how Della survived on the Moon without oxygen and food. However, considering that the McDucks were going to space in the first place, having no backup supplies in case of helmet damage is nothing short of folly.
The rocket was not supposed to be launched for at least another month and the rocket was completely untested, so the rocket neither having any backup supplies or backup gold makes sense, Scrooge didn't believe that Della would find out about the rocket that early and would steal it for a joyride before it was even tested.
Della not being able to blow out the boys' birthday candle makes some sense considering that Moon has no atmosphere and Della is getting her oxygen from the Oxy-Chew, thus there's no air she could use to blow the candle. Although that begs the question how the candle can burn in the first place
It was probably an emergency flare with its own internal oxidizer. Though that raises other questions, like why didn't she use that to signal with when the McDuck search ship had passed overhead...
So Della's been transmitting this whole time to Earth and yet no-one's received them. Why? Because the Moon Planet inhabitants were hiding themselves with cloaking technology. It'd stand to reason that this could've including blocking out any transmission leaving the surface.
Della's characterization shows traits from all three of her sons:
Like Huey, she is an enthusiastic Junior Woodchuck.
Like Dewey, she has an impulsive, daredevil attitude.
Like Louie, she recognizes narrative patterns (like how she may become Fire-Forged Friends with Penumbra).
While Donald is her brother, she most certainly shares his temper when frustrated.
No wonder Scrooge almost went bankrupt when sending so many ships to search for Della; they're fueled with GOLD. Normal rocket fuel is expensive enough; using gold, plus the obvious expenses from making the ships, would be a huge funds-drainer, even for Scrooge's fortune.
Of course it does seem out of character for Scrooge to use a far more expensive fuel for his rockets, unless using gold has it's own benefits. Maybe gold is actually far more efficient and thus actually (slightly) more cost effective?
Gyro's explaination in his notes beyond it not being something evil (yet) is that Scrooge has alot of it. Which sounds dumb, but actually would be very useful as far as keeping it in Mc Duck Enterprise's control since it's experimental and dangerous technology. Fueling it by gold means that only the richest duck in the world could reliably power them. Well as far as anyone on Earth knew anyway.
The far side of the moon turns out to be hollow, but made mostly of gold. The high density of gold would compensate for the missing mass, combined with the tidal locked orbit, would leave the true nature of the moon undetectable from earth.
Part of the reason the Moon people might be having so much trouble with the Mites is because their weapons are made of gold. Gold is, while heavy (under usual gravity conditions) a soft metal that would be less effective made into sharp edged or pointy weapons like spears.
There weren't exactly a lot of sharp objects near Della when her leg was trapped in the wreckage. Either she found another way to get the debris off her leg and only had it transformed afterwards, or...she had to BASH her leg off with a blunt object.
Or she could've used the glass from her helmet? Maybe a sharp piece of metal broken off?
Still pretty horrifying considering that, unless she got a big and heavy enough piece she would have had to saw her leg off rather than just chopping it off. Unless of course instead of actively removing her leg, it came off more naturally due to gangrene, which isn't much better.
The Moon Mite is revealed to be stealing metal because she's trying to keep her baby fed. The city of the Moon Planet inhabitants is filled with gold, the Moon Mites' preferred food. Since the Moon Planet cops have been actively hunting the Moon Mite, she probably wasn't able to forage for food properly, meaning that she (or worse, her baby) could have starved to death.
It's clear that her Apocalyptic Logs also serve as her SOS transmissions, which was her first method of getting rescued. Throughout the first year of her disappearance, Scrooge had been burning his Money Bin to look for her. How did her transmissions not get through back to Earth?
It's possible that the equipment used to transmit the messages back to Earth was damaged in the crash, and Della hasn't figured it out yet. Or she knows, but still makes the transmissions as a coping mechanism to keep herself sane.
It's been established that Della went into space before the triplets were hatched. How did she know what gender they were going to hatch into? She clearly states that the lullaby was something she sang to them before they hatched.
Doctors in this universe probably developed ultrasound that works on eggs.