Fethry is a counterpart to Huey (both being nerdy and quirky Junior Woodchucks) just like Gladstone is a counterpart to Louie. Their color scheme follows this, as Fethry and Huey both wear red, and Gladsone and Louie both wear green.
This would also make Donald the Dewey's counterpart, with Donald being a semi-retired adventurer and wearing a blue outfit in most animated continuities.
Fethry also establishes that Junior Woodchucks often keep their guidebook under their hat. Since Dewey and Louie aren't Junior Woodchucks in this continuity, that explains why they have no hats.
A later episode also establishes Della to be a Junior Woodchuck. And as one, she also wears a hat (specifically, an old-fashioned pilot helmet).
Dewey and Louie are probably Junior Woodchucks as well, but less enthusiastic (and probably not going to activities anymore), since in "The Day of the Only Child" Huey partners with Burger and Bouncer precisely because Dewey and Louie aren't there (and even comments on how Dewey would have not tied the knots so well).
In order to avoid getting in trouble with their uncles, Huey and Dewey decide to blame Louie for stealing the sub. Why? Louie is the only one who can talk himself out of trouble.
Fethry claims he's "Uncle Donald's favorite cousin". Which probably isn't a lie. Because as much as he might think Fethry is "Cuckoo Bananas", at least he's not "The Worst" like cousin Gladstone is.
Why is the seabase falling apart? Remember the first time we saw Scrooge in the pilot? He was in a meeting with the board and one of the items was cutting funding to 'unnecessary departments'. One of those departments was deep sea exploration. This leads to some Fridge Horror...
In combination with the previous episode Scrooge told Donald there he is in Scrooge's will with the implication it is a nice piece of the estate. Donald expresses shock he is even in it, but considering Scrooge's younger kin, it makes perfect sense. Consider the known living younger kin that have been seen thus. The triplets are not mature enough or smart enough to safely handle the responsibilities of this much wealth. Gladstone, while he is lucky, is very selfish in how he never uses the luck to help others. He skates by solely on luck, which Scrooge doesn't personally approve of. He respects a hard worker. And in this episode, Fethry shows he is a hard and diligent worker, but not the capable of people in leadership. Donald, however, traveled with Scrooge for years on adventures, showed intelligence and hard work. He showed leadership during Magica's recent attack by leading the family in a coordinated offense, recognizing each one's skills and where they would best attack from. Donald also took the responsibility of parenthood for the young boys seriously and raised them well but never sought Scrooge out as a financial crutch. He pushed his own self-reliance. Sure, some of that was their falling out from the Spear of Selene incident, but Donald stuck to it for over 10 years. In short, Donald has shown himself to be a smart, capable, and responsible person Scrooge can trust his money to when Scrooge finally dies.
It also fully gives an idea of why Donald, on the rocks with Scrooge, would go to him for emergency babysitting duties. Gladstone is the worst and a bad role model while Fethry is a bit off kilter if well meaning. Also underwater.
Despite Fethry's oddness, there's no way he got the job just because he's kin or that Scrooge wanted him far away - that's just not how Scrooge works, because rusty though it may be, the lab's still an investment. So, Scrooge sent the one person he knew that he could actually trust with maintaining the lab, and has literally no one else in his organization who could do so for as long as Fethry has. As further proof, it's already been demonstrated (with Gyro, Quackfaster, Launchpad, etc.) that Scrooge clearly has no problem with keeping a Bunny-Ears Lawyer on staff when they're legitimately good at their job.
Putting employees in a dangerous situation they can't handle is not an option for Scrooge, so Fethry probably had the requisite skills for maintaining a fifty-something year old deep sea lab to get assigned there.
Fridge Horror
It was Scrooge's Board of Directors that cut funding to deep sea exploration. Fethry's lab was in a bad place because of it and he nearly died as a result. Let's also remember that the Board would later show itself to be very petty (see Bombie) even before they were revealed to be leading F.O.W.L. Did they cut funding with the idea that Fethry would be caught in the crumbling lab and thus kill a Duck family member? After all with Scrooge's habit with Fethry's calls and episode one malaise he could have easily missed the part that Fethry was calling about something like 'OH MY GOD OH MY GOD EVERYTHING IS FALLING APART'.
