The audibly-clockwork demon is not convincing at all! It is definitely concerning, because anything new and unknown is a threat, but that's not what they're going for, here.
"You remember what I said before about generalizations. But as for what the crusaders would expect - they're always direct. They know what they want and they immediately take it by force. They're smarter than the average mortal, but they don't use trickery or complex stratagems if they don't think they have to. They advance if they expect to win, retreat if they expect to lose, don't do stupid suicidal charges or go into a mad rage like some demons in battle."
"If they want to harm mortals they just torture or rape them, not - mind-control them into loving a demon, or trick them into breaking a vow and losing their god's favor, or leave them alive to wander the earth bemoaning their woe for a hundred years. They have mind control magic, but they don't use it for kicks, unlike most other demons. Oh, and they have tongues at will, meaning they can speak and understand all languages, which would explain why we can. Assuming the Miliways effect works on things we hear through the door, that's worth checking."
"So if some incubi wanted to hold this house, and they were confident they could withstand an attack, they probably wouldn't bother talking to the crusaders at all, except to show them how strong they are and how pointless an assault will be. And I'll be there and can help sell it. The crusaders will probably assume the escaped slaves are in here with us, too."
"Two or three incubi, with me helping, could easily hold this place all night against three average crusaders, or even indefinitely, because demons don't need to sleep. Unless their mystery third person is a strong paladin or cleric, in which case all bets are off. If they try forcing their way in, you can swarm them with clockwork demons, but if they have good offensive magic at range, we'd have to use the guns. Can you disguise them to look like really powerful crossbows?"
"By the way, besides alignment detection, a cleric is likely to have detect magic, which lets you see magical auras and even identify spells and magic items if you're really good with it. It's an orison, so they'll use it if they have it prepared. And an incubus would have a magic aura, on account of the at-will tongues, but I can't show you what to mimic because I don't have tongues myself."
"We forgot to describe the demons' magic! This was very stupid of us. Not all demons can do everything, same as mortals, so you can just pretend not to have any of those abilities, but you should at least hear about them."
"Some incubi can read minds, usual range of sixty feet. It doesn't always work, people can throw it off, and it only reveals fleeting surface thoughts, but it's useful for detecting invisible or hiding people. Some can also talk to their allies telepathically. They can enchant people into being friendlier to them or doing something they suggest, but only reasonable-sound things, not betraying your allies or anything like that. Unless you're the kind of person who thinks switching sides in the middle of a fight is reasonable. But the main thing is that they can change shape at will, looking like any humanoid species and even imitating specific people they've seen."