I noticed the potential advantages of a partial performance of the Becoming ritual myself, and discussing them a bit with the Author. It's his opinion that you can postpone completing the ritual, but doing so indefintely may have negative consequences (as in, you get mad; interstingly, that's what one of the rumour about Quendalon's eventual fate says it happened), but he wouldn't enforce this strictly; if player and ST agree on it, the character may use partial becoming as an immortality method for centuries on end, and so indeed Caitlin-like characters may be possible.
Partial Becoming has its relevant advantages as an immortality method: Transforming the Body is not a permanent magical effect stopping aging, but a partial transoformation in an inherently immortal creature, so no ongoing accumulation of Warping. OTOH, you lose access to fatigue, which for a mage focused on spontaneous magic (as many Merinita are likely to do, since their attitude to magic seems rather free-form, check Virtues like Glamour, Animae magic, and Faerie-Raised Magic) may be devastating. Hence, any mage that undergoes this would be wise also to seek Mysteries initiating (or self-initiate) in Virtues like Diedne Magic (which now, with the opportunity to self-initiate, may be gotten as completely disjunct from actual links to druidic heritage), Faerie-Raised Magic, and Potent Magic in one's specialization(s). I wonder whether a character that has Transformed the Spirit, too, might expend an extra amount of faerie might points in lieu of a Confidence point.
One may or may not choose to Transform the Spirit, precociously, too. There are relevant advantages (completely immune to Warping, so it's the most airtight immortality method available so far; you get Faerie Might for penetration and magic resistance, you lose Gift's social penalties) and drawbacks (spells are now aligned to the faerie realm, with its harsher penalties in divine and infernal auras; faerie might points must be expended to power spells; mage is subject to faerie wards; character loses access to Parma Magica).
What a smart character should postpone as much as she can is to Transform the Mind, given its crippling penalties to learn new Abilities, do spontaneous magic, and learn new spells. It's never worth it. IMO completing the transformation should be the hallmark that the character has exausted any zest for life, exploration, and research, and is ready to retreat to Arcadia.
Given the free-form, unstructured nature of Merinita mysteries, it might acutally be possible for a bright, precocious young mage, only a few years out of apprenticeship, and totally eschew the need for a mainstream Longevity Ritual. Even more so since Merinita Mysteries, differently from other House Mystery Cults, do not have strict Merinita Lore thresghold requirements for initiation. Yet another demonstration that spontaneity pays at enlightenment: take this, Criamon 8)
Whether it would be advisable to do so depends on whether the character would find the features and consequences of becoming more of a solace or a burden. We should not forget that other, non-House Mysteries detailed in TMRE may offer other immortality methods which may be of comparable benefit to Becoming and might be potentially accessible and ideologically compatible with Merinita ideology: eg. Alchemy and Theurgy Mysteries are especially likely to do so. Certainly Becoming looks on average more interesting than the only known alternative immortality methods, Immortality of the Forest, with its crippling residency requirements, and the Criamon Body Path, with its unsufferable behavioral cult-like restrictions. It is especially interesting to characters with access to Virtues that help offset its restrictions to spontaneous magic. Other Merinitas might prefer self-initiate Unaging, or look to non-Merinita Mysteries, for prolonging their lifespan.