Thanks! When I started this thread, I didn't realize I was stepping into a matter with some controversy attached .
In any case, I do believe that the corebook does state that the Gift only allows you to learn Magical Supernatural Abilities. Mine is not the first printing, so I don't know if this was a modification, but page 63, the "Ability Types" reference sheet says in the last sentence under "Supernatural Abilities":
So that part is clear to me. Where it becomes murky is when we get to Learning Supernatural Abilities. As I understand it, Erik's interpretation is that only Supernatural Abilities from the same Realm penalize Supernatural Ability Source Quality, whereas Richard's (which was how I originally understood it) is that all Supernatural Abilities penalize Supernatural Ability Source Quality. Page 166 of the corebook does not say anything about what Realm, it just says "Supernatural Abilities" without any further qualifier. But, it is referring specifically to characters with the Gift learning Supernatural Abilities, so it could have been assumed that all the abilities in question were Magic-aligned. However, while as I quoted above The Gift only allows the learning of Magical Supernatural Abilities, a character could still have, say, "Sense Holiness and Unholiness" from the Divine Realm and use The Gift to learn a Magical Supernatural Ability. In that case, it is not clear to me whether Erik's or Richard's interpretation is the intended one.
RoP: The Divine says, on page 47,
It does not say specify "Divine" Supernatural Abilities subtracting. I like Erik's rule that the Favored Abilities would also not be subtracted from other Advancement Totals for Supernatural Abilities, but I am still not clear about whether the penalty is Realm Specific.
Also, I had a couple other questions on somewhat related issues pop up while reading. It seems that learning Supernatural Abilities and the favored ability mechanic are the area of the game that I am still having trouble with. What Realm is the Ars Notoria Supernatural Ability associated with? I would have assumed Divine (Since its in the book) because p. 100 says "The actual powers of Ars Notoria are considered Divine for determining how they interarct with other realms." But, it requires the Gift to learn, suggesting it is Magical, and p. 98 says that learning from the text is difficult for advanced Hermetic magi, implying that their Hermetic Arts are subtracted from the study total (which, under Erik's interpretation would also suggest it is Magic-aligned).
Finally, I recently acquired a copy of HoH: Societates, much to my delight, and while I haven't read it yet, I was flipping through the book and on page 106, there is a spread on Favored Abilities. The rule here is different than the one in Divine. For Ex Misc traditions, it says that learning Favored Abilities is not penalized by other Favored Abilities, but is still penalized by other, non-Favored, Supernatural Abilities. This is different from the text from Divine that I quoted above, which said that for Holy Traditions, all Supernatural Abilities did not penalize the learning of a Favored Ability. This isn't really a problem, but I just wanted to know if the intention was for Favored Abilities to work differently in different contexts, or whether it was ideal to have a unified mechanic for them.