Forcas (RoP:TI p.61f) provides Might 40 example stats for a Watcher, and can impart Corrupted Knowledge (RoP:TI p.64 box). All Watchers are pretty powerful, but if you wish to start with a lesser demon plaguing the character more directly, Forcas can also 'gift' him with a Famulus (RoP:TI p.53 box, and p.54f) first. So your plot can scale over the campaign.
A Watcher's plot might indeed be, to trap some Titanoi between hammer and anvil by teaching them Corrupted Knowledge. If the Order finds out, it might march the corrupted Titanoi, thereby driving them into the widely opened arms of hell. And while the Order doesn't know, the Watcher's victims are corrupted anyway day-by-day.
Well I see them as simply a different branch of the same tree. The two are not exclusive, as it were. A Goetist can also be a Theurgist, and vice versa. These all also mix well with Hermetic Spirit Magic and Spell Binding from Mysteries. I think there are reasonably Titanoi lineages that emphasize either aspect of their magic, being more fully or less fully integrated into straight Hermetic Magic and more or less affiliated with the infernal or infernal powers. Which to my mind doesn't even mean whether they have/practice Summoning/Goetia. Remember that Goetia works on all realms of spirits, and that it isn't the only version of Summoning style Art available. Indeed there are multiple versions of Summoning that function in dramatically different ways.
There are (Realm) Summoning Arts, such that can summon creatures affiliated with a particular realm, and/or there are (Entity Class) Summoning Arts like Goetia that can summon specific types of creatures ( Spirits in the case of Goetia ). There is also (Elemental) Summoning and Sihr. There are interesting possibilities in combining some of this material into more fully developing out a hedge tradition for the Titanoi to dip into for non-hermetic effects if you were interested. The Faerie Bargainer material from ROP:F for example has 'Bonding' which allows for bargains to be struck that allow the use of a Faerie's powers by the mage. It wouldn't be unreasonable to me for the pre-hermetic tradition of Tytalus to have done something similar with spirits.
Anyway... rather than continuing down the non-hermetic road as it were, I just wanted to point out that Hermetic Theurgy isn't totally distinct from that, I view it instead as the outflow of integrating the pre-hermetic tradition partially/imperfectly into hermetic magic and creating a mystery path by which to retain some of the elements of their tradition/lineage. I'd recommend your player if they are choosing to go the hermetic theurgy route also take a look at the aforementioned Spell Binding and Hermetic Spirit Magic.
@ Vortigern: IĀ“m quite sure, that there is a distinct difference between Hermetic Theurgists and Goetic Summoners. It starts with this...:
Hermetic Theurgists: "Daimons are difficult to summon, but with this spell a maga can call the Daimon repeatedly until it hears her call and sends an Aspect of itself to answer her. This does not command the Daimon, but most will perform a specific service when summoned [that is: voluntary]. Many Daimons will perform unusual services, beyond simple spellcasting, and often beyond the limits of Hermetic magic."
Ars Goetia, Summoning: "To begin, the character must draw a circle upon the ground. [...] It [the summoned spirit] must remain there until he dismisses it [...] While the spirit remains in the circle, it cannot affect the sorcerer or anything else outside the perimeter with its Powers. Often an angry spirit waits invisibly within a hastily drawn ring until it erodes or collects enough dust to break the line, and then exacts its revenge upon the sorcerer or anyone else nearby upon being freed."
In my eyes this is more than a different branch of the same tree. Are you friend or foe of the spirit youĀ“re calling? I think, the question asks for a decision, not for a compromise. This is a different tree, for me.
There is nothing saying that a Theurgist is inherently friendly or respectful of the spirits they summon. Nor is a Goetist inherently domineering or hostile. One can still bargain with spirits even if you know how to compel them. And Theurgy even includes mysteries for making it easier to compel and/or attack entities like Synthemata. I don't think the stark philosophical divide that you have in your perception of the two is anywhere near as clear or distinct as you seem to think. I'd agree that it is there as a general tendency in the design of each path of spirit magic, but not that it is inherent in how any given practitioner is going to look at their magic or their relationship with spirits.
And that is still just speaking of the 'feel' of their interactions with spirits. When I say they are two branches of the same tree, what I really mean is that they are both magical practices/techniques that are both descended from the magical tradition of Guorna the Fetid, and then through Tytalus to the Titanoi. That partial integration of the magic of Guorna by Bonisagus resulted in the current Hermetic guidelines allowing interaction with spirits, and a furtherance of this gives access to Hermetic Theurgy/Spirit Magic mysteries to the Titanoi, allowing them to continue along their path and still be Hermetic.
That doesn't preclude a given philosophical viewpoint to someone aligning their build one way or another, as a current Titanoi with theoretical access to any of these mysteries.
One could argue that a philosophical divide likely exists however between the magical tradition as it was practiced by Guorna and before, and how it likely was practiced and continues to be practiced by Tytalus and the Titanoi. Though given the nature of the culture of House Tytalus and the Titanoi, I'd find that being a purely friendly interaction at all times rather unlikely. That is one aspect of of the Tytalus in general that I think leans them towards being open to multiple methods of interacting with spirits. Bargain, when necessary or advantageous, or when you feel like it... but assert oneself and be unafraid of conflict when necessary or beneficial as well.
I think, I did say what I have to say. In my eyes the difference is obvious. We can talk about gradual nuances (you just started with it), but the difference will stay, in my eyes.