1) Of sex scenes and impractical action suits, is there a reason other than fanservice to add them?
I have used sex exactly twice in Arcana Dium. The first was a rather explicit little scene that I'm sure has driven off atleast a few potential readers.
That said, it (combined with the next 2-3 chapters) helped explore just what sort of relationship one of my MC's had with her boyfriend. Being that this was very early in the story (Chapter 3, onwards) I don't see the problem. It's shocking. Good, it's not a children's story, and it's not your average relationship.
The second time wasn't a closed doors, or even merely intimated, it was spelled out, "they had sex". I think they were even (almost, I think I used "made love") my exact words. It was exactly one sentence long, the whole sex scene. Why/how? The purpose of the chapter was to show a disconnect between the two people involved.
Now for the admissions, the earlier scene was fanservice to one fan who I also consider a good friend, and who was the inspiration behind it. It was also service to myself, I needed to write that scene. It was just a happy coincidence that I could work it into my story.
2) How much in your experience (if any), do such scenes derail or delay the actual plot and how can one use deus sex machina without it appearing as a writer on board?
*cringes at the bad pun*
Sex is a powerful subject. Sex sells. Sex derails my thought train rather often and at the most inappropriate of times, and delays my re/actions towards what I should actually be focusing on.
The real question was the last paragraph written from my view, or the view of plotline? I'm not entirely sure if it's the plot or the author that get's distracted, but that's how it happens IMHO.
3) When having to choose between a century-old introvert vampire MC and a secondary character who is a sorceress, hardcore extrovert and barely legal, who would you pick to write such scenes about?
My MC (the one involved with the sex scene atleast) is 19. So from that point of view, the sorceress, even though age is about all they'd have in common. Personally, the vampire appeals to me more. Call me crazy.
4) What do you do when readers want such scenes but it doesn't fit with the plot?
Simple: Beyond grammar and spelling, I rarely listen to my readers.
Reasoning: Among other things, they've suggested that my MC is being mind-controlled, when she's already in a weak enough position as it is. I know my characters better than my audience does, and my characters know the plot better than I do. Why should I trust my audience when I'm already telling the story second-hand?