But if Cassandra can use templar powers without fear of Lyrium addiction, then you don't need lyrium for templar powers. First DAO showed you didn't need lyrium for templar powers, then DG and the comics said you did (which makes much more sense honestly), and now we don't again?
Yes, but this is only an issue if you're reasoning under the assumption that we have all the details, when, evidently, we don't. There could be various factors behind why Seekers don't use lyrium, as I saw mentioned somewhere here earlier, perhaps a semi-magical ritual of some kind, not dissimilar to the Gray Wardens. It may be dangerous, or require extensive preparation, but enables the subject to perform Templar talents indefinitely.
And as you can imagine, if Templar talents are achievable without lyrium, there’s a good chance the Templars are unaware. Seekers are most probably bound to an NDA, of sorts, but there's no doubt a lot of rumours and conspiracies surrounding the matter. I mean, lyrium's function in this case is not to enable the talents, but is to control the Templars. Chances are, making the talents require lyrium upkeep was actually what took all the serious thinking.