Do I have to read the extra chapters?

I would say yes, it offers insight into why Ranon did what he did. Also, it has critical information about the kingdom's "curse"

Also, SPOILER NOW:
.
.
.
.
There's some type of reincarnation going on??? Like, Holston and Hernando show up?? But in Hernando's case it looks like it's just an ancestor who looks just like him and shares the same name but with Holston it's not as clear. Also, LUCIEN is named???


Anyway's yes, absolutely read the side story.

Thank you dear