Ruling from Alex Werner, FFG Game Rules Specialist via the official rules question form:

Question: If I commit Plan of Action to a skill test during the Mythos Phase, would it only commit for one wild icon?

Answer: No; if you commit Plan of Action during the mythos phase, you would give it 1 willpower and 1 agility icon.

Rulings from Alex Werner, FFG Game Rules Specialist via the official rules question form:

Question: Let’s say that Trish Scarborough is at a location with another investigator and there is an enemy engaged with that other investigator. If Trish discovers a clue at the location and uses her special ability, can she “automatically evade” the enemy engaged with the other investigator? If so, what exactly happens - is the engagement broken and the enemy exhausted?

Answer: Yes, Trish Scarborough can use her ability to “automatically evade” an enemy engaged with another investigator. Once she does so, that enemy will exhaust and break its engagement with the other investigator.

Question: Let’s say that Trish Scarborough uses Damning Testimony to discover a clue at the location of an enemy and then uses her special ability. Can she “automatically evade” the enemy even if it is not engaged with any investigator? If so, is there any effect other than exhausting the enemy?

Answer: Yes; it’s possible to automatically evade an enemy that isn’t at your location. That enemy will exhaust, even if it’s not engaged with another investigator. And while nothing additional will happen with Trish and the Damning Testimony, Trish could trigger an effect that required her to evade an enemy, like Pickpocketing. (Note, she can only trigger effects that don’t require a “successful” evade).

Ruling from Alex Werner, FFG Game Rules Specialist via the official rules question form:

Question: Let's say that Carolyn Fern has Peter Sylvestre in play and Peter has one horror on him. Then Carolyn draws Rational Thought. Will Peter heal the horror on him at end of turn?

Answer: No. Rational Thought makes it so that Carolyn Fern cannot heal Peter Sylvestre through his reaction triggered ability.

Have you ever played a Mystic deck built around recurring Premonition every turn?

Book of Living Myths is just a persistent Premonition attached to an Asset for Kōhaku Narukami decks. Assuming you keep an eye on the number of Blessing / Curse tokens and have ways to manipulate the Blurse distribution in the chaos bag, Book of Living Myths can guarantee that you draw at least 1 symbol and at least 1 Curse / Blessing token of your choice per turn. That has some unique deckbuilding opportunities:

For cards that only care about "Symbols" being revealed (assuming you use the Taboo list), like Jewel of Aureolus, Read the Signs, Hypnotic Gaze, or Ritual Candles, Book of Living Myths guarantees that you always satisfy the symbol conditions.

For cards that actively require a Blessing or Curse token to be drawn, like Eye of Chaos, Ofuda, Beloved, or Fey, then Book of Living Myths can help Kohaku reveal Blessings / Curses consistently where other Blurse Mystics would need to play Favor of the Sun / Favor of the Moon + Olive McBride to match him. Most Blessing and Curse cards are balanced around the fact that you're unlikely to consistently reveal them from the token bag when you need them; assuming you build your deck to take advantage of both Blessings AND Curses, Book of Living Myths removes that random chance altogether to the point where it almost feels like Kohaku is cheating.

This seems broken and undercooked. Insane value for what it is. I imagine this would make sense if it could be used only on enemies with trait Humanoid and the test difficulty would be dependent on that enemy's attributes.