Douglas would be thrown out of the Diogenes in the first ten seconds: he's a smooth talker, very eloquent actually, and is much too fond of banter to be that unclubbable. However, Douglas is with MJN rather than a proper airline because he got caught doing his dodgy smuggling stuff once too often. Not drugs or anything, but he does like to make a quid taking alcohol, chocolate, orchids one memorable time, across borders.

So I think it's more likely that Mycroft has engaged Douglas for one or two very discreet, low key smuggling operations and has a bit of a hold over him. They probably like each other well enough, but Douglas will have no illustions over who is in charge. It's likely Mycroft caught a glimpse of Martin once when he was at Fitton airfield and just took note of his (smaller more ginger) resemblance to Sherlock, just in case. And now in case has happened. He would leave it to Douglas to make sure Martin knew his lines, and Douglas may not question it, as long as he was certain that no harm would come to Martin as a result. He's obviously a little fond of the squirly captain, even though the squirly captain can be very irritating.

And I think Martin, once kidnapped and deposited in the family grounds, might take refuge in knowing the lines and delivering them. It would save him from attempting actual conversation, which he always screws up.

I imagine Mycroft has asked for 'handling' advice from Douglas too. THat would involve making sure that he remembers that Martin is Captain Crieff (so many people assume Douglas is the Captain, even though Martin's the one in the Captain's uniform. Even when it's pointed out, nobody believes it). So soothing Martin's ego about his rank, his skill and so on might help keep him in line. I can kind of see Mycroft very smoothly indulging in this.

Hope this helps.