I certainly know about the existence of an extra level, but I would like to clarify a few things.
1) Does the Mastery Amulet only work for the Hero's primary skill (i.e., in most cases, their faction skill), and does it not apply to other skills in a normal situation, thus having no effect? (I'm just clarifying the practical usefulness of the artifact.)
2) So, you simply edited the necessary spells for the extra level and added the creature to the spellbook with the extra level. Does this mean it's quite simple?
3) Some creatures also have abilities with Mastery. Since they are added to the spellbook, can you change the effectiveness of some of these abilities in a similar way? (Have you tried this?)
2) I explained everything above; you just write "extra" in the mastery characteristics, and add a parameter to the spell file.
3) I don't know about abilities; it seems like there's only one level of mastery there... maybe you can make the damage of a Titan's Lightning spell vary depending on the mastery level