Altarrin is very clearly holding back quite a lot of potentially very important pieces.
...That's fine. He's probably right if he thinks it's safer for her not to know. Ellitrea is a Mindspeaker and can shield her thoughts and detect compulsions on her, but that doesn't make her immune to non-magical forms of leverage.
There are plenty of options for mages, and most of those who responded to the initial emergency are among the potential candidates; this isn't a coincidence, obviously, but it's convenient.
For the last thing... She should put Ledia on it, the girl is loyal enough to Altarrin, and disinclined to ask too many questions about the weirder aspects here – and she's a young nobleborn mage just recently out of the Hall of Learning, who knows the ins and outs of the social cliques among the young ambitious mages at court (who, with mage-sight, are most likely to notice something in conflict with Altarrin's cover story for Carissa.) But she's also nineteen, and, well, Carissa won't like her, she's - not frivolous exactly, but not nearly cynical enough...
Betril would do better on that front, except that he's not that deeply loyal to Altarrin, and also not a woman...
Merda, maybe? She's not Gifted, but if anything that's a benefit, she's by default below the notice of anyone who is. She was one of Altarrin's protégés, which is why she didn't come to mind first - it's a potential hint for watchful eyes that Altarrin is playing at something here - but that was twenty years ago, most of the watchful eyes have short attention spans.
And Merda is a good pick in other ways. She made her way to the capital by scoring highly on the civil service examination, despite her lack of noble blood, and she - may have had her fumbles at first, but if anything, it might just have taught her a deeper cynicism about the Empire. She's certainly organized enough to keep track of everything that Altarrin wants to know.
And her loyalty - isn't totally unquestioned, she might betray Altarrin if it were in her interests, but it's been twenty years and she hasn't done it yet.
Once she's passed some brief updates to those of Altarrin's mages who already know that something happened yesterday, Ellitrea summons a page and sends a note to Merda, asking her to please urgently bring some records from the Archives to a particular mage's office.
(It's a code. The mage in question is one of Altarrin's people, he works on sewage-purification spells and is of little interest to anyone, and the unusually thorough shields on the Work Room are because he runs tests on potentially disease-causing raw sewage – if anyone thought to check, which they haven't, it's a coincidence that they also block scrying.)