You will get a lot of different answers from us on this one, because there are a lot of legitimate takes on how to play it.
We also have to be careful to avoid copying rules outright here : This forum is not a way around buying the books and supporting Ars Magica and its creators.
So, I'll try to be as clear as possible, while not giving away any details.
Remember Rule number 1:
Its a story, and the storytellers make the rules.
Everything should come secondary to creating a compelling and enticing world. As long as you're upfront with the players on how things work, modify things anyway you like.
Now for your questions.
First, most of them are answered directly in the books. You'll just have to read them over again, and get used to them over time as you make characters.
Let's take them in order:
- Yes, you can have Nature Lore AND Faerie Magic. Explicitly.
All Merinitae have Faerie Magic. Period. You get it free at character creation.
Nature Lore you can rule either gets taken as a virtue, or can be taken as a "cheap initiation" - basically a free virtue for a set price of time, yielding the ability.
In fact, any magus can theoretically develop Nature Lore, (see the Source book for the Rhine Tribunal : Guardians of the Forests).
2.) Again, explicitly yes. Within Merinita there is a group called the Wayfinders which mixes and matches the various Mysterie in various ways. The Merinita mystery schools are the most common progressions, in that they just plain make sense as progressions.
You'll notice that mysteries folow a theme. That's intentional. Mysteries are more than paths to power - they are the commitment of one's power to a specific... ideal is the wrong word, but its close. Create a compelling "ideal" or "idea" for a path to follow, and the SG should rule that its possible to follow it.
3.) Outer and Inner Mysteries are just what they sound like.
Mystery Cults have "paths" or "progressions" of development and understanding. On a basic level, the outer mysteries allow you to progress "inward" to the deeper mysteries.
The Bjornaer Heartbeast is the clearest example of this.
The Exoteric Outer Mystery is the Heartbeast. This is far more than the ability to turn yourself into an animal; it’s the understanding that you are as essentially your totemic inner “beast” as you are human. This “heartbeast” is the core of your magic and power and, in a real sense, of who you are.
Then there are the Inner Mysteries. These are powers which can be brought forth. As an Animal, you learn to communicate and interact with the world on a more primal level - not merely through intellectualized words, but through the wolf’s howl; through plumage or marking with scent.
Thus Sensory Magic. Without the fundamental understanding of the self-as-beast, this beastial form of magic is impossible, which is why Bonisagus could never integrate it.
Then you get to the Innermost Mystery - the Inner Heartbeast. This is a deeper and more fundamental understanding of the Heartbeast- that it is not merely an Animal, but the ideal of that Animal and what it represents. This opening of the truest self is only possible once you have journeyed long down this path of understanding.
Mechanically, its one thing leads to another.
- Woah Boy. The mechanics are the toughie. Again, I'm trying to avoid plagarism.
There are four elements to an Initiation:
The Ease Factor (“Target”)
The Mystagogue
The Script
The “Bonus”
Mechanically, every virtue will have an Ease Factor, or Target, just like any other feat in Ars Magica. There are six possible numbers.
Three for Minor, Moderate or Major Virtues if you have a leader who knows the virtue and script.
Three for the same if you have a script, but are initiating “blind.” You can initiatie yourself, but only blind.
Your Mystagogue + Script Bonus + “Bonus” must add up to the target in order for the Initiation to work.
Any initiation needs a Mystagogue - a leader who leads the ritual. The Mystagogue adds his Presence + (Cult Lore) to the Initiation Total.
Then there’s the Script. This is the ritual component of the initiation.
Basically, in order to get a new Virtue, there’s a lot of different stuff you CAN do to “power” the ritual or initiation.
Each of these things has a value. These values are pretty clearly spelled out in the introductions, and several times in the chapters, for redundancies sake. Taking on a Major Flaws are worth X points, Taking on a Minor Flaw is worth Y points and so on.
The total value of all the stuff you do is called your “Script Bonus” (from the Initiation Script.)
Finally there’s are possible extra bonuses.
The most common is a bonus for previous sacrifices. Taking on Ordeals (which means suffering new Flaws) gives extra bonuses for later initiations.
Then there are “Sympathetic Bonuses” - these are little extras thrown in for flavour from time to time, and are entirely optional and depend on the mystery at hand.
SO Final mechanic is this:
Mystagogue’s Presence + Mystagogue’s Lore + Script Bonus + Bonuses from previous Ordeals and Sympathies = Initiation Total.
Does that help at all?
Leaving a cult is certainly a major story hook IMS.