So, she would actually have to [strike]invent two spells[/strike] invent a spell and invest an effect in the Talisman, rather than invent an uncastable spell.
The effect would be a Level 35 Rego Herbam (base 35, R: Personal, D: Momentary, T: Individual), to teleport the talisman to her hands. Although, apparently, Magi of Hermes has a Rego Terram guideline to "transport a target instantly to a place to which you have an Arcane Connection" which is Level 25. Which I prefer to level 35, personally. Although I kind of like the idea of having it only being usable by her (to keep someone from grabbing her talisman and sending it to Mount Doom), making it a 38.
The spell would be, say, a Creo Imaginem, base 1, R: Arcane Connection (+4), D: Momentary, T: Individual, for a final spell level of 5. Although it also occurs to me that it could be Rego Imaginem, base 15, R: Personal, D: Momentary, T: Individual, for a Spell Level of 15.
Installing the effect in her Talisman looks like it will take two seasons: Lab Total of Int 1 + Magic Theory 6 + Rego 10 + Herbam 10 + Inventive Genius 3 + Paul 4 + Sheelagh's Int 2 + Sheelagh's MT 3 + Sheelagh's Inventive Genius 3 + Rego Aura 10 + Lab General Quality 2 + Lab Specialty (Enchanting Items) 1 + Lab Specialty (Rego) 1 + Lab Specialty (Herbam) 1 +5 Talisman goodness = 62. She could have it usable 24 times a day (+5), or 6 (+3) if we go the only unable by her route, and not have it take any longer. Or, she could Experiment (gaining another +1 Lab Bonus), and have it done in one season, with it being usable once a day, barring a really crappy roll on the Extraordinary Results table. (And, frankly, if her Talisman is getting swiped more often than that, she deserves to not have one anymore. )
Her Lab Total for the Imaginem effect is Int 1 + Magic Theory 6 + Creo 6 + Imaginem 5 + Inventive Genius 3 + Paul 4 + Sheelagh's Int 2 + Sheelagh's MT 3 + Sheelagh's Inventive Genius 3 + Aura 5 + Lab General Quality 2 = 40, vs. a CrIm target of 5. Or, if it's Rego, her LT would be 50, vs. a ReIm target of 15. Either one would probably entail her coming up with another low-level Imaginem spell to invent.
Numbers are all based on her and Sheelagh's current scores, and this is at least a year and a half off anyway.
But does this sound about right?