For the second part, you are forgetting the modern possibilities of hiding the face and even altering the voice. With modern instruments (a computer not too basic, for the speed, and software, for which there is even free versions), it can be some postprocessing, but it is not necessary to be a mega pro expert to do it, remembering also that the customers of such video are not interested in a show off of Hollywood blockbuster special effects, they are more interested in the "fapping effect"
:-D .
Crystal seems rather the type that would not be bothered by the situation Myriam+Marc - aside that for what she knows, any eventual reference to the fact of Myriam being the mother during the video (that anyway would appear only in the incest patch, not in the normal game), could be role playing.
And if they become such good friends with Myriam and Crystal understands something, it could on in the line of the "complicity between friends".
For the first part, is where I would more agree, but that said, they way they spoke (or Zorlun made them speak), Crystal does not do it only for the money, she does it because she likes sex and the exposure.
It would not be the first case of a professional porno actress that does also videos that are not a film and are more improvised.
Even because "nowadays" (already from many years, I would say) the majority of porn is not movies with at least some serious pretention of a story, and the majority of the porn content is spread (and sold) through Internet, and most sites will have short videos, not long movies.
Plus, if you prefer, you could put it as another scenario,
Myriam making a gift to Marc, or accepting a request from Marc, of hiring Crystal for the threesome.
Some porno actresses will get offence at being considered or called prostitutes, but others had prostitution (or "escort with sex", in theory "escort" could be even without sex and would be required to have also a certain style, but that is not said) as a step before doing porn, or even doing "escort with sex" in parallel with the porn actitivity.
And if Crystal and Myriam have such a good friendship, Crystal could do it for Myriam as a favour.
Not saying it will happen, I had myself never considered the possibility, and maybe even Zorlun had never thought about it. But as you see, there is more than one reason and way in which it could happen without making it break completely the logic of the story.
So, it's up to Zorlun.