Although I still don't understand Isabel's decision, as she stubbornly ignores her royal duties, trying to pass them on to someone else.
Formally, she never married Nikolai. Consequently, she was not a queen, but seized power by force. A rebellion began against her with the aim of placing Andrew Gryphon on the throne. Apparently, Isabel decided to relinquish the throne in order not to continue sowing discord.
Since the Gryphon dynasty was interrupted, it would be fair if their closest relative, namely Frida, ascended to the throne.

