In "Abu Dhabi", Carolyn is trying to cut any cost she can as much as she can. The cargo hold heating is very expensive, as Carolyn has previously pointed out to Martin. Why doesn't Carolyn make Arthur put the cat in the heated cabin with him?
Possibly Carolyn doesn't trust Arther to keep a cat alive. And it can't go in the cockpit, since we learn in "Ottery St. Mary" that Martin is such a stickler he won't even allow a *hypothetical* otter up there.
None of the three Birling Day episodes actually end with Douglas successfully getting the Talisker off the plane: in "Edinburgh" Martin unwittingly gives Birling the bottle which Douglas has hidden it in, in "Paris" Douglas gives it up for a thousand-pound reward from Birling and in "Timbuktu" Carolyn is attempting to steal it from him, but Birling is never actually given the whisky. So what happens when Douglas does actually get the whisky off the plane, as apparently happens in every other year? Mr Birling seems completely unaware that such a thing happens in "Paris".
According to Douglas in "Paris", he sells it.
Moreover, why do they act like Martin lost the £100 to Carolyn in "Paris"? They convinced Carolyn it hadn't be stolen and Mr Birling got his whiskey and Carolyn doesn't talk to him anyway. Martin should be due is reward.
Directly after the events of the episode "Fitton", Douglas's wife Helena confesses to her affair with her Tai Chi instructor, and Douglas and Helena's marriage proceeds to break up. This information is not revealed to the other characters (or the audience) until the later episode "Limerick". So, for the entirety of the episode "Kuala Lumpur", Douglas, who is a recovering alcoholic, frequents a series of fully-stocked illegal bars around the airfield while going through a difficult time in his personal life and surrounded by people who are under the impression that he is a happily married social drinker.
In "Gdansk", Douglas seems uncharacteristically aggressive towards Martin's jabs about pretending to be a captain for Helena, and proposes several deals which would forfeit Martin's right to tease him over it. On first listen, this is seemingly because of his own guilt over lying to her, or dang, even just over his own pride being wounded. Not until the reveal in "Limerick" do we realise that, chronologically speaking, "Gdansk" likely takes place not long after he came clean and Helena revealed her affair and their marriage collapsed...
As of the penultimate episode, Carolyn, Arthur and Martin all have happy endings waiting for them in Switzerland, and giving up MJN Air is in all of their best interests. Douglas, on the other hand, given his age and his previous track record, will be left without a job and almost unemployable. His interactions with Herc show him becoming unhinged at the prospect of having to answer to someone other than Martin, so even if he landed another job, it's highly unlikely that he'd keep it for long (Heck, even assuming that he did manage to keep one, he'd have to be on best behaviour the whole time and would probably wind up bored and miserable. Goodbye, zany schemes and flight deck games). The way he talks about his social life hints that he is very rarely honest with other people about his troubles- Martin was probably the closest thing he had to a true friend. So, he'll be a former alcoholic losing his job while going through a divorce, with no support network whatsoever...
Douglas seems to still have a fixation with alcohol, even though he cannot drink it. He steals Mr Birling's whiskey, tries to con Carolyn into giving him expensive wine, he goes out of his way to help maintain an illegal social drinking culture at the airfield, and pretends to drink even when the social context doesn't require it (see: his "vodka" in Fitton, his first cabin address in Abu Dhabi). We all know how fond of games and challenges Douglas is- perhaps he's just playing "How long can I stay on the wagon?" with himself- and as usual, he seems to be winning. But what happens when he gets tired of playing?