I only allow Assists in Narrative time if both characters are actively using their skills; i.e., there is nothing else they can do. In some cases, I disallow Assists, for instance, if one character is playing a card game, no one would allow a bystander to give them hints in the game. Similarly, if only one character is in position to Notice a detail, no Assist is allowed. Mostly, you want your players to feel there is a mechanical benefit to working together, so give them a benefit when they think cooperatively.
Only by using the Oxford Method can a caster extend his casting time past a single turn with the Increase AP immuto. Even in Narrative time, I enforce these restrictions, for the most part. I would actually allow a character who uses the Oxford Method to 'exploit' the Increase AP mechanic in Narrative Time if it served my campaign; because the Guild is always on the lookout for such mages, using thier spells in some cases very dangerous. Interrogate Magia could be gamebreaking, but honestly, a mage who really dedicates themselves to such a single purpose kind of makes sense.
As Fatemaster, you will either have to impose some restrictions on using Increase AP in Narrative Time, or find a creative way to dissuade your players from kidnapping everyone they see for extened interrogation sessions.