Galadriel will be, as usual, infuriatingly cryptic. She has learned exactly enough about Thedas to be distrustful of Templars and Wardens on principle alone (nobody's mentioned seekers yet so sorry, Cassandra, you get to count as a Templar); the more heavy-handed Cassandra gets with her interrogation the less helpful Galadriel will become. Her accounts will be truthful but probably somewhat lacking.

If you need somebody to express a somewhat alarming (burgeoning) knowledge of how one might attempt to open a rift, I can cheerfully have Galadriel drop that sort of information on Cassandra. Also if anyone else who was present would like their characters' lives made slightly harder, I can have Galadriel insinuate that she suspects she's not the only one who could figure it out.