Are the Merinita magi dupes/pawns/tools/slaves of faeries? Or are they simply magi heavily interested/invested/tied in faeries?
And Quendalon is truly what he claims he is? Or is he indeed a vile fae impostor who has manipulated, corrupted and transformed the House to suit need and design of his species and inscrutable machination of his queen?
I've been pondering this one myself. Depending upon where my players decide to settle in my soon-to-start Rhine saga, the forests and faeries could well be a very important theme. I like the Huntress cult and their ambitious aim to get revenge on Quendalon so I'm very tempted to make him the "vile fae imposter". The vast majority of Merinita magi would be honest and uninvolved in this particular faerie plot (as I don't buy into large conspiracy theories and it's been quite some time since Quendalon changed the direction of the house) but one or two key magi who are still working towards the plan of Quendalon would be likely to be antagonists.
Quendalon is the man who changed Merinita from 'cool hippies' to 'damned madmen; in my mind. Don't get me wrong - I do like the Merinitas. I am of the opinion that the Merinita house, as a whole, isn't actually an ally of the faeries. The individual Merinitas are probably allies often, but looking at some of the Lore of faeries, and the Lore of the house, I have to view Quendalon in a malicious light. I sort of view him similar to the God Learners in Glorantha, a man who's unravelled many of the secrets of the faeries, and thus has power over them. The House popularly is likely quite friendly with the faeries, but the deeper you go into the mysteries, the more you realize the faeries aren't people, and instead are tools...
A long time ago (Ars 2nd ed), we played a saga where Merinita House splitted in two, House Merinita: close to Nature, dropping their focus on Faerie and House Quendalon which of course focus on Faerie.
I realise that it was made possible because there was not too much emphasize on "not bringing harm to your sodales", because a House with a focus on Faerie will interact with them, with a high chance of creating troubles. Very likely, the Quaesitors would have intervened or even forbid for a House to endorse publicly the study and interaction with Faerie. After all, it is not so different than Diabolism.
When in ArM5 canon history of the Order Quendalon took over House Merinita in the early 800s, "the Quaesitores" were sceptical Guernicus and his disciples. Guernicus let Tremere build his power in the Order, and, when 817 the Grand Tribunal instituted Certamen as 'decisive in all disputes' (HoH:TL p.38f), he gave up on the Order and withdrew to Magvillus. His successor Fenicil in the 840s plotted to have Tremere assassinated.
So a power play by Guernicus against Quendalon over the purpose of House Merinita would have been most unlikely.
This allows many different takes on the nature and motivation of the Quendalon 'having two bright rubies for eyes' (HoH:MC p.77), that returned from the Bohemian forest to retake the House. He certainly followed his agenda (see HoH:MC p.78 and p.92ff Arcadian Mysteries) energetically for well over a century, though - and the House still shows his imprint.
Cheers
The HoH:MC book intentionally does not say. Here is my favorite take:
First, I'd throw away the idea that faeries don't exist when humans aren't around. Faeries are generally malevolent, Vitality-hungry spirits, that scheme and work to seep vitality from humanity and Magic. The highly-cognizant faeries do so consciously, the uncognizant do so sub-consciously, with an animal-like cunning more than true intelligence, but all faeries hunger for vitality and constantly seek to obtain it. They are all at heart Nothingness given form, always seeking to fill their internal void by stealing Vitality and existence from the real world, but never succeeding to fill the emptiness that is their essence, erecting only a facade of false Seeming in its stead.
The Merinitia magi are mostly useful and unwitting pawns for the Faerie Queen that overtook the House. Since the time of Quaendalon, only magi on the path of Becoming have been Primus, and they direct the House to serve the Queen's interest. This consists mainly in increasing her power over other courts, including other Faerie courts, through various deeds of subterfuge and illusion; she is especially wont to receive tribute, even symbolically, from other courts. Most Merinitia magi don't serve this purpose directly, but their initiations into mysteries of stories and phantasm starts their path towards becoming more Fay and part of the Queen's court, as well as providing them tools to manipulate courts. The House leadership and a few select individuals work more directly for the Queen, arranging tribute towards her, usually by indirectly siphoning tribute through the House or a Tribunal to courts which, in turn, pay tribute to her. These are mostly magi on the path of Becoming within the House.
That said, there are magi in the House, or formerly in the House, who do not serve the Queen but are, rather, free agents. They are armed with great understanding of Faerie, and they generally use and manipulate faeries. Some may suspect the House's leadership still works for the Queen, but they usually don't really care. These include the Followers of Pendule, who follow their own whimsy, and the dreaded Dream Witches, who follow their own twisted agenda, as well as an assortment of magi following their own personal goals.
YR7, I'm not going to use your cosmology myself, but for anyone who is, here's a little bit of colour, for a mystic to add, ripped off from Campbell.
There are only four paths the hero can take.
He can fail and be ripped apart by demons
He can prostrate himself before the Creator Father.
He can marry the Divine Mother.
He can realise he was, himself, a god all along.
These are clearly the four Realms. Thus the Faerie Queen is a major deal, cosmologically speaking.
Ave sodales. Apologies for resurrecting this ancient thread, but I think this is part of the never-ending story, and since I have a different take on the question, I thought I'd share it with you:
In my opinion, Tweet and Rein-Hagen, when designing Mythic Europe, got inspired by Dungeons & Dragons' alignment system. It's easy to spot the dichotomy between good and evil in the Divine and Infernal realms, but is it not likely that the Magic realm represents (platonic) order and therefor the Faerie realm stands for chaos?
Chaos not to be confounded with destructive or deviant, but in the unpredictable, the constantly mutating, and thus ever fascinating way. Faeries are not devious; they are rascals. They don't lie to commit fraud; they modulate the truth to create confusion. They don't steal all your gold. That is just boring. They steal the ladder underneath you to keep you stuck on the roof. And they do it because it entices a reaction. They are addicted to change. They don't just kidnap a baby; they replace it with a changeling and curse the parents to not notice the difference. There's no malice involved. To them this is a source of years of interesting happenings.
If you go with this, then Merinita magi, and anyone really who spends long periods among the Fey, will grasp this, either cognitively or intuitively. If they want to stay in various faerie's favor, they need to indulge this chaotic behavior. Feed it even, by providing sources for change. And condone when they are the occasional target of some strange situation.
So, whether Quendalon was transformed or replaced, as soon as the truth is discovered and revealed, he will need to change again, probably into something even more bizarre. Storyguides are not only free to come up with whatever explanations they like; it is their prerogative to make these explanations contradictory and as confusing as possible.
There are however two new issues that need to be resolved. If there are four quasi orthogonal dimensions intertwined with the reality that is Mythic Europe, then there should be overlapping regiones between some of them. For instance, there must be good chaotic regiones as well as evil chaotic ones. As a matter of fact, just like characters in D&D need to situate themselves somewhere on both axes of alignment, it is conceivable that there are no pure realms in Ars Magica. They all should have some gradient of either good/evil and order/chaotic. (If the influence realms have on each other is alluded to in ArM5, it must be noted that a gradient nature of alignments within regiones is probably a departure from Ars Magica canon).
And from that stems the additional problem that all spells necessary to detect or travel to a particular realm or regio, might need to be somewhat redefined as cross-over spells. Not to mention how more complicated the Realm Interaction Table becomes to determine aura modifications.
To be used at one's own peril. Alas such is the way of the Fey...
I do not recall seeing anything in the Lion Rampant-era Ars Magica books that would particularly support such an interpretation, but it has been a long time since I re-read them.