There's a discussion of Spell Mastery with regards to Rituals in the Ars forum.
https://forum.atlas-games.com/t/spell-mastery-and-ritual-spells/6698/1
I've made a number of comments, and I believe I had been clear on my belief here, but the interaction with jebrick leads me to conclude that I either haven't been clear, or that discussion is buried somewhere and too much trouble to hunt for.
I'm looking for troupe consensus to go one way or another. I think I make it clear that a Mastery ability score of 1 in a Ritual doesn't allow one to avoid the risk of botching Ritual spells, and it's RAW. I ask all players to give considered answers to this and keep in mind that I'm not advocating we go by RAW. I'm saying what I believe RAW is, and it aligns with what I've always understood. I have no vested interest[1] in this decision.
The choices are ritual spells are always rolled with a chance of botch, unless a combination of mastery and virtues drop botch dice to 0, or a Mastery Ability score of 1 in a ritual allows one to cast a ritual in a relaxed[2] manner.
[1] It alters Apollodorus' reluctance to cast the AoH spell a bit, doesn't make it consistent within the framework of the saga to date, but it's a minor quibble.
[2]Note relaxed manner is open to some level of adjudication at the time the ritual spell is cast. I typically have given a lot of latitude for relaxed as it relates to formulaic spells. I probably won't be quite as lenient with allowing the player to determine whether it is relaxed, and if the matter is contentious, I'll put it to the entire troupe before deciding.