I'll explain to the best of my ability but I'm sure someone could do it better
do "arcane warriors" and "knight enchanters" actually exist
Arcane warriors?
Yes. They do, they were an ancient order of elves who were body guards for the elven nobility.
Knight Enchanters are basically a play off the Arcane warrior art. All they did was call it something else and "chantry-fy" some things to perhaps make them safer.
Are there actual "spells" or do mages simply use their will to channel and then shape the energies from the Fade into what effects they desire?
Mages use their will to shape the fade energies. So, you're right. There are no incantations but I'm sure there is novice to advanced technique to shaping the energy into whatever you want.
Are mages feared because they are extremely powerful or only because they can be possessed by spirits/demons (or both?
Both. Basically, the Tevinters instilled the fear of magic into the mundanes after what they did. Blood magic used freely and without restraint, blood baths of slave slaughtering to fuel powerful rituals and spells. Rivers running red with blood while demons roamed to do bidding (or at least the magisters thought) for their masters. And for a mundane to dare speak would cost them their lives with the bat of an eye.
It was a terrible dark age and that is why people fear those who have magic. Magic is not evil but it can be used to do so verrrry easily. People fear the fact that a mage can destroy a village with a whisk of the hand if they so choose. (Even though many mages can barely light a candle).