True, he never says that in so many words, although one would presume that the reason he never became part of the Seekers, nor underwent the Rite was because it wouldn't work on him (or rather, would work too well), making it pointless?
My impression was he was simply going to retire once they became the Seekers, but was convinced to go on "one last mission"
Do you know the exact dialogue for that? I don't have the DLC but from the videos I've watched I don't remember Ameridan mentioning that.
We know that the idea behind Tranquility being used on mages was inspired by a mage trying to join the Seekers, and failing to attract a spirit (thus remaining sundered from the Fade). I haven't seen anything conclusive that suggests mages cured of Tranquility are permanently deprived of their magic, especially when Seekers themselves gain magical abilities after their vigil. Cassandra and Lambert also state that the cure is dangerous because recipients become emotionally unstable and are thus more vulnerable to possession, which shouldn't be a danger if they are cut off from magic and the Fade. Of course, this could have just been a lie (or ignorance on Cassandra's part) to cover up the truth.
Emotional instability is definitely a risk. But there shouldn't be a risk of possession, as Seekers are immune to that.
What would be a risk for a mage would be turning into an emotionally unstable person with the power to tell physics to sit down and shut up 