Here's my take on the matter:
(1) There is a difference between possession and joining. In the latter case, the spirit or demon doesn't take complete control of the human, but the two beings join willingly and each brings something into the union. In the former case. the demon takes over completely. The results of a joinng can be good or bad, but needn't result in the typical chaotic behaviour of so-called "abominations". Considering the type of human who'd willingly join with a demon, the results are probably unpleasant but the actions of such an entity may not necessarily be distinquishable from those of an unpleasant but normal human.
(2) As I see it, temptation by a demon can be successful whenever the victim isn't quite sane. A sane human would never agree to the type of bargain a demon usually offers, since it involves letting the demon in with no strings attached. The single plausible instance of such a thing in the DA games and stories I found in "The Last Flight". There are moments when you think you'd give anything in order for a particular outcome. This may be in war and on the verge of losing, or if you're desperately in love, or any other situation of high emotion. The trait the demon represents must be present and be so strong that it threatens to overwhelm your rational faculties. Subtle demons may try to inspire such emotion over time by various means, some of them magical in nature, but they can't take over until then. If a mageborn is overconfident enough to agree to let a demon in with no strings attached while apparently sane, then this is likely a case of pride overwhelming their rational faculties. No cases have been reported where such a thing ended well, so it's not rational to think you're the sole exception and bet your mental freedom on it.
(3) Mage training to avoid possession will necessarily include mental disciplines to keep your sanity in situations of high emotion. I'd say this is a kind of training anyone would profit from, but for the mageborn it's a necessity. I see parallels with Jedi training in the Star Wars universe.
(4) Tranquility will be applied to those mageborn who are unable to acquire these mental disciplines, regardless of their innate magical power or talent. Being emotionally unstable to start with is probably a risk factor. Considering typical human development and the fact that magic usually manifests during puberty, mage training is probably rather difficult. Tranquility may remain a necessity unless a different method of locking a mageborn away from demons is found.