You just described every PC mage in DA. Just making a DA mage shatters the establish lore of the world as you are impervious to possession.
I think the DA team can be excused to not include that if it isn't an important plot point somewhere. After all, there is no guarantee that any one mageborn will ever suffer a possession attempt, even less a successful one, and in addition, it would just be a nonstandard game-over like sleeping with Morinth in ME2. It's different with Corypheus: if he fought you and you were vulnerable, it wouldn't make any sense at all not to have him make the attempt, unless there were precautions you could've taken.
Maybe DA2 should've had such an option in Feynriel's mission, but again, if the outcome is just a non-standard game-over it would be a small and insignificant bonus outcome. Compare that to the Sloth demon in DAO taking everyone into his realm and making you fight for your life. That was a nice idea, you weren't immune to that influence, and you weren't even so super-special: Morrigan noticed what was going on right away, and if I recall this correctly, was waiting for you to link up so that you could fight your way out together.