I never said that guest character stories are not canon, it's not up for me to decide. What I said is that they do not set up, but based on a main story instead. I'm not sure what you meant by "Lycalis and Viridice are also considerably more alive in the guest routes", because I've never seen any story that would go further than involving a guardian of that kingdoms in it. They are mainly focused on a guest character, as they were meant to.
None of them uncover any info about Viridice or Lycalis that we couldn't know from main plot. The only thing I can remember is Zanna talking about flying monsters (wyverns?), how they can be used as food, that they attack in big packs, etc, but nothing about Keisiema for example. It's true that Margarita's Inn is a good setup for a story arc, but is not a merit of a side story. In fact, Cia's story starts there thanks to the main plot. Same goes to MC's demon-phobia. He has one because of what happened in the main story, not the other way around. Also, I'm not sure if MC is afraid of demonsses specifically. He surely was traumatized by Kendra, but I don't remember him peeing his pants when he met her again or Evalin for example, yet when a nightmare stroked him infront of Maryen (maybe she is the reason to them btw), he was afraid of her too. I think it's more like a giantess-phobia, fear of someone so big and strong compared to him even if they mean no harm. Maryen is not an angel btw.
Yes, you are right, ideas from side stories may be used on main one (I never doubted that), ther's nothing wrong about it, and yes, I must admit that bloodstone could be among those. However, if we assume that there is a chance that Maryen becomes corrupted enough to try reviving her dad, which would be pointless at that point, she might simply resort to necromancy magic instead of using runes.