After being told that he'd hurt himself trying to swim through money, Louie points out to Scrooge that he can swim through money just fine. Scrooge retorts that he trained himself to do it and had to build up the muscle for it first. Given the feats in the comics and past cartoons (which this show is heavily based on), it'd make sense that a guy who can destroy a steamboat with his bare hands can also swim through metal with ease; he's just shrugging off how big of a feat it is.
Of course the Number One Dime exposed in the reliquiary is a decoy: with Magica after it, it's perfectly normal that Scrooge keeps decoys around.
Also, the comics already had someone accidentally spend the Number One Dime — if it happened here too, Scrooge won't keep the real one around where it could happen again.
There’s hardly anything about Della in Scrooge’s archives. It's likely that after her disappearence, Scrooge removed all records of her out of anger and/or guilt.
The plot of this episode gets kicked off when Louie needs an extra ten cents for soda that should normally cost a dollar. Of course a cheapskate like Scrooge would charge an extra ten percent at the vending machine.
Despite saying that she was cooped up in the mansion all the time, Webby has clearly been to the Money Bin with Scrooge a few times, as Quackfaster recognized her and told her that she was not getting into the Archives in such a way that makes it clear this is hardly the first time that Webby has made the request. Scrooge has been allowing Webby to tag along with him for some time now, and not just letting her live in the mansion as Beakly's ward. The only reason Webby hadn't seen more adventure before is because Scrooge was effectively retired.
Scrooge boasts that he doesn't spend a penny more than is necessary (and if you remember Magica from the original series, $15,000,000 for magical defense might seem like a bargain), but then you have to account for the fact that McDuck Manor has "a dozen or so" spare bedrooms. Why would a confirmed bachelor like Scrooge need that many rooms, which would have surely cost more to add to the mansion, if he's so thrifty? Simple: prior to the Spear of Selene incident, Scrooge was very big on family. Donald and Della adventured with him frequently, and he had portraits of his own family from Scotland all over the place. He may well have been looking forward to having a large, extended family living with him before the incident ten years prior that effectively had him go into retirement.
Also according to Word of God, Donald and Della's parents (Hortense and Quackmore) both died when they were still fairly young. It's quite possible that Scrooge had to take in the twins at some point and they lived in the manor with him until the Spear of Selene incident.
The Archives at the Money Bin still use a card-based catalogue despite computers being much more effective. Someone as obsessed with her job as Emily Quackfaster would not allow a computer to do it for her.
It's also likely that Scrooge, who is old-fashioned and generally adverse to new technology, wouldn't want to spend money on fancy new computers when the old card-based catalogue works just fine.
Why does Scrooge keep Emily Quackfaster and Gyro Gearloose on the payroll when they're both clearly a few cards short of a deck? Because, as in the comics, Quackfaster is extremely loyal to Scrooge and is his longest standing employee, having been brought on board by Scrooge's sisters decades ago. As for Gyro, he had a grandfather in the comics who was Scrooge's first friend in America, and to whom Scrooge feels he owes a great deal. Gyro's employment with Scrooge is probably entirely by grace of that legacy.
When the vultures complain about Scrooge buying a $5,000 velvet pillow as an unnecesary spending, he explains that it is for his first dime, which is too valuable for him. But at the end the dime is revealed to be a decoy, he carries his real first dime with him at all times. So the vultures are actually right on this one.
On the other hand, what would convince people the decoy is real more than a pompously pricy pillow?
Scrooge tells Louie that he could crack his skull if he jumps into the money bin without the proper muscle training. Louie later does fall in and isn’t seriously injured, but it clearly hurt regardless and he does have trouble moving through the coins. The same thing almost happens to Gyro when he nearly gets tossed in during Lil’ Bulb’s rampage, and you know a twig like him would have probably broken every bone in his body.
