Time Sensitivity
The cardinals have been sequestered for almost two weeks. While most Romans would like a timely decision, none expect that they will hurry their deliberations, and the cardinals could argue for several more weeks before reaching an agreement. Magi may think they have time to return to their labs and invent a spell to address a specific case that comes up in play. But there is no guarantee that the cardinals will remain undecided. In most cases, the story does not allow for player characters to leave and return later.
However, this shouldn't be a problem for truly powerful magi. Spontaneous spells and copious amounts of vis should allow any magus to participate in this adventure, ensuring that a magus does not need to be a Mentem specialist or have expert knowledge in the political workings of the Church or the city of Rome. Liberally using vis should also overcome the Divine aura penalty imposed on spell casting. Remember, however, that using vis when casting spells adds to the number of botch dice, should a player botch a spellcasting roll.