First, that AotH isn't a Formulaic spell; it's a Ritual spell.
What is that second spell? Is that a ReVi maintenance spell? The ReVi effect doesn't extend the duration, just keeps it from ending early. Is that a MuVi spell to increase the Duration? Would that work on a non-fully-integrated spell? It looks like the former since you're talking about casting it afterward and wrote "maintain."
Yeah, it is a Ritual, but based on my understanding all the Riuals are a sub-class of Formulaic spells. Right? There is no spontaneous Ritual
Yes, it is a ReVi maintenance spell. Nothing prohibits you from using the guideline to
invent a spell with a longer duration and maintain a spell for longer then its own Duration. MAINTAINING THE DEMANDING SPELL is just an example, and I think you can invent spells with other R/D/T.
You wish to use a Faerie variant of ArM5 p.162 Maintaining the Demanding Spell with a D: Until, right? What such a variant works on - just specific Faerie magic or also standard Hermetic magic - is at least a serious troupe decision.
Simply using this on a Ritual that is itself not fully integrated with Hermetic magic like AotH (see p.161 last chapter), for which changes of parameters require still major breakthroughs, IMO is leading at best to utter failure of the Faerie spell, and potentially even to disaster as the Aegis resists it.
The guideline referred to is "sustain or suppress a spell" which says nothing about extending the spell with ReVi. You might be able to use MuVi to alter an aegis spell to have an until duration however.
Actually, Rituals are a separate class from Formulaic. This is critical in a few cases. This is why you'll see so many notes about allowing both Ritual and Formulaic stuff, some having been added to the errata due to having been overlooked.
Canon wise it doesn't work that way. Aegis is special and to make it work you need a special version of the Aegis which would be duration Concentration. I personally think the Bonisagi have been working hard to create such a version and may be close to doing so as it would allow all members of the Order to have access to a more potent Aegis at the cost of more risk and more vis to cast the Maintaining the demanding spell which many alliances would likely be okay with. If you can secure/sponsor that version of the Aegis then yes, your Until duration would be game changing.
Alternatively, you could directly have had Merinita invent an Aegis with an Until Duration. Also possible with the help of powerful Fey.
But... Until duration has always sit sideways with me. Just offsets the games so much. Here is how I keep it interesting. It is a fairy design and attracts the interest of the Fey. As such Fey find it funny, challenging, interesting, are binded to, painful, etc... whatever floats their boat really and they get involved in triggering the Until condition. Might even cause Fey to be born with the only purpose of triggering the condition and the longer/powerful the effect, the more numerous and powerful the Fey.
All interesting avenues if implemented properly... bends a bit cannon as you need a major hermetic breakthru to have happened which as per cannon, has not yet happened in 1220.
I remember being in another conversation about Maintaining non-concentration spells.
Essentially, what I recall to be the result, was that the word Maintain implies a certain level of activity, and that since our only example is for use with Concentration spells, it would appear that this guideline is for use with D: Concentration spells only.
By this logic, what you're looking for is in fact a MuVi spell to transform your spell to have D: Until (rather than D: Year), which is fairly easy, since they are the same 'level of effect'.
OMG... and just like that, virtually all Aegis across the Order are now being assisted by a Merinita MuVi to Until ritual. Does it even have to be a Merinita or can anyone with the labtext learn & cast the effect?
No more vis wasting. No more Yearly gathering to see if we go into twillight. Just an increase in Fey meddling.
Can a regular magi mimic the Until duration with a +1 magnitude? Might be worth it in this case.
Yeah, I think in general, even if it were theoretically possible (my personal opinion is that it isn't), the impact this spell would have on Vis economy and everything surrounding it puts it in the realm of "Transforming Mythic Europe", aka something the saga should to some degree pivot around.
A counter-argument against it being problematic, or against it working in this way?
If the latter: Aegis is a specific Ritual, NOT a guideline. You can take it, or leave it. Or, as the core book points out: "Inventing a version with different parameters, however [as opposed to same parameters, but different level], would require a similar [major] breakthrough, and years of research"
Absolutely. And I wasn't suggesting tinkering with the Aegis ritual itself. Indeed, nothing in what I was suggesting involved Inventing a version with different parameters (or even a somehow changed version with identical parameters. No invention necessary, strictly speaking, except on a MuVi spell to tinker with durations. That spell would be the same one used to, eg., change spells between D: Concentration and D: Diameter. Remember, MuVi affects other spells, allowing you to change existing spells to suit the situation.
In fact, p. 93 of the core book tells us that...
while p. 159 of the same venerable tome tells us that:
... and thus (because there's no change in the final level of the effect, some would argue that this is a superficial change, though said MuVi spell would need to have D: Sun (or similar) to accommodate the long casting of the Aegis-ritual.
I certainly would not consider it to be a superficial change. Changing the Duration is already a significant change, and the guideline for significant change doesn't say equal to a magnitude, but less than or equal to one, so it is supposed to be able to handle zero-magnitude changes as well.
Problem is that Muto Vim "spells can only be used on Hermetic magic, as they depend on a good understanding of the processes involved".(p.159) Aegis is not fully integrated so it cannot be manipulated by MuVi spells.
Back to my original question: Rego Vim Sustain GL can be interpreted in a way when "sustain" includes extension. Sustanance over time.
Rego Vim does not prohibit using it on Rituals, but - mainly bc setting reasons - Aegis is not fully integrated. Ergo you can easily argue about effects based on Sustain GL are not useable on this special, not fully integrated spell - which is not even a guideline as you mentioned.
There is no exact rule against a Sustain effect with Until duration. I think it is more of a decision of the troupe based on one's taste, and not a strickt rule in the core - or any other - book.