So between the core rule book and the House books, we know who did the following:
Spontaneous casting -Diedne
Familiars: Merinita
Parma - Boni
Aegis -That Guy (Serfs Parma)
Enchanting (and implied Talismans) Verditius
Naming the Gift - Jeribon
Twilight - Ciramon
Certamen - Tremere
Hermes Portals - Mercere
Order Laws - Guernicus
Order Democratic Structure - Tytalus
Flambeau was strong and added some fire magic, and Bjornaer was powerful, but not sure what she added other than some nature magics, maybe a unified Animal form... ?
However, I never saw anything about who brought the Longevity Rituals to the Order. It makes sense that Tytalus would in some ways since he was a necromancer and fleeing a master that wanted to use him and others for life entension magics.
Any other thoughts on who came up with the Longevity Rituals?
You forgot that Bjornaer it's told that she helped specially integrating theirs Senses Ranges, it's speciall said on their Chapter of HoH: MC.
Jerbiton did other things too: added many utilities and principles about Mentem and Imaginem, naming the Arts like Arts too.
I can imagine many of Rego based upon original Tytallus and Tremere's tradition.
Longevity Rituals could come from the decimated priests of Apollo, it's posed on Ancient Magic. Tremere and Trianoma did it.
When Trianoma was forming the Order, she approached various magi who in turn travelled to Bonisagus to work with him on establishing magical theory. Tremere, Tytalus and Pralix were not on that list, being apprenticed to the necromancer Guorna the Fetid - a necromancer who was, it turns out, preparing one (or all) of her apprentices to be the vessel in which she bound her life to prolong it.
It was through her discussions with Bonisagus that the integration of the longevity ritual happened. Her subsequent death at the hands of Tremere and Tytalus put the two younger magi into the Order in her place.
(nobles parma on all of this, but I'm pretty darn sure I've read this as canon somewhere)
Of course the Tremere, Tytallus and prallix vicotry over Guorna is true, but that original begining doesn't change the fact that the Founders of two houses are Tremere and Tytallus. Were Guorna &/or their Apprentices the inventors or integrators of Longevity Rituals?
That can be, but Chtonian an Necromantic Magic still without being the only with tricks to extend the life expectancy, i am thinking on the Apollo's Magic, the Folk Witches and the Elementalist Refining, three very different Traditions with his own and near methods on practices but no reasons.
I like the Guorna explanation. Not sure if it's cannon but it makes sense.
Other possibilities.
Mercere apparently had some affinity for creo maybe he knew how to extend life. Though he left heirs so he didn't use it on himself. And what where he and Boni working on when his Gift got damaged. It would also make sense that the original cult of mercury had some version of the longevity ritual, being all about getting together and chanting about self improvement. Of course if it was already established as part of Mercurian magic then...
Why couldn't longevity rituals have already been a part of Boni's original tradition. The guy was busy and he got a lot done so it makes sense. Also if the stories about Trianoma and Deidne and Merinita are true, it seems the guy got busy a lot to. But there's not a lot of stories of heirs (or maybe there are mythic blood and all) so maybe he was already shooting blanks when he started meeting Magas and asking them if they wanted to come back to his cave and learn some real magic. Of course even if Boni didn't already know it when he started then it's probable that...
Boni developed longevity rituals along with hermetic theory. Makes sense we know from Rival Magic he did corpus research to break the limit of energy. He probably worked out some of the other basics of healing and life extension earlier on. Maybe once he worked out creo and corpus an LR was a natural by product.
IIRC Mercere was a kind of natural magician and expert in Muto potions. It makes sense that he integrated a lot of Artes Liberales and Magic Lore (items of virtue) into hermetic theory. That can include ways to use ingredients to extend life.
Flambeau brought penetration to the OoH, IIRC. But I might be wrong. What he was was one of the most active recruiters of other magicians into the OoH, and that is a major feature in the creation of the Order after all.