Uumm... given that a mage is hounded by demons, I'd say he is.
Demons for the most part don't even notice regular humans. Mages are like becaons to them. So no, even a Templar stuck in a Fade wouldnt' be at the same danger.
And when mages cast magic, they draw from the Fade. They have at that point a connection.
Theoreticly, a demon could posses them then too, altough it's not as likely.
*facepalm* You are ignoring established lore in favor of Chantry dogma.
Allow me to share the Nature of Spirits with you, as told by the Chantry.
Nature of Spirits
According to the Chantry, the spirits of the Fade are the first children of the Maker. He turned his back on them because they lacked a soul—they could twist the Fade to their liking, but lacked the ability to imagine and create, and thus emulate their creator. The Maker created a new realm, separated from the Fade by the Veil, and this realm would be one that his new children could not alter at will. These new children had the spark of the divine within them, and the Maker was pleased. Supposedly, ever since the Maker created His new children, the spirits from the Fade have watched humanity with curiosity and, in the case of the more malicious spirits, envy and desire.
The malicious ones among the Maker's first children were jealous. They called out to the mortals, drawing their sleeping minds across the veil and saw the land that the Maker had created for them in their dreams. They coveted the spark within them, but did not understand it. They shape the Fade to create the land that they see in the minds of men, and seek to draw the spark from them without truly understanding where it might actually come from.
A spirit cannot comprehend a world that is static and immutable and is not familiar with the concept of time. It cannot comprehend what it means to be within a body of flesh. Some spirits cross the Veil because they desire something they can never have.
They observe the mortal world through our dreams. Now how do they view our dreams? Not just mages here. Because the Fade is a realm of dreams, that all non-dwarves go to. Even non-mage humans, elves, and kossith. They all go there every single night.
The Fade
The study of the Fade is as old as humankind. For so long as men have dreamed, we have walked its twisting paths, sometimes catching a glimpse of the city at its heart. Always as close as our own thoughts, but impossibly separated from our world.
The Tevinter Imperium once spent vast fortunes of gold, lyrium, and human slaves in an effort to map the terrain of the Fade, an ultimately futile endeavor. Although portions of it belong to powerful spirits, all of the Fade is in constant flux. The Imperium succeeded in finding the disparate and ever-shifting realms of a dozen demon lords, as well as cataloging a few hundred types of spirits, before they were forced to abandon the project.
The relationship of dreamers to the Fade is complex. Even when entering the Fade through the use of lyrium, mortals are not able to control or affect it. The spirits who dwell there, however, can, and as the Chantry teaches us, the great flaw of the spirits is that they have neither imagination nor ambition. They create what they see through their sleeping visitors, building elaborate copies of our cities, people, and events, which, like the reflections in a mirror, ultimately lack context or life of their own. Even the most powerful demons merely plagiarize the worst thoughts and fears of mortals, and build their realms with no other ambition than to taste life.
--From Tranquility and the Role of the Fade in Human Culture, by First Enchanter Josephus.
"They create what they see from their sleeping visitors." That doesn't sound like a mage using blood magic or lyrium to enter he Fade. That sounds like me having a dream one night and the spirits and demons found it interesting.
And if you say mages are dogged by demons and are a danger at all times of becoming possesed, to that I say tosh, or poppycock. There's only one kind of mage, one, who is truly in as much danger as the Chantry claims. And those are somniari.
Every other mage MUST use lyrium or blood magic to enter the Fade consciously. A somniari on the other hand, is unique in their ability to enter the fade without either.
And here is the definition of a dreamer, taken from the dragon age wiki.
A dreamer, or somniari in Elvish, is a mage capable of entering the Fade at will, without the aid of lyrium. A talented dreamer can shape the Fade and affect the dreams of sleeping people, killing or driving them mad. However, dreamers attract demons and most prove too frail of mind to survive a demonic possession. As a result dreamers are rare.
The elves of Arlathan were supposedly talented dreamers, and there are stories of ancient Tevinter magisters who were able fend off rivals by haunting or killing them in their dreams. Adralla, the mage who invented the litany, is also known to have found a defense against dreamers
Now taking away a Dreamer from the equation, we have standard mages who absolutely MUST use lyrium or blood magic to enter the Fade. Jowan can send us into the Fade with blood magic. Irving can send us in with lyrium. Neither can do it without either.
Gaider has said that in order to become an abomination, a mage absolutely must do one of two things. Lose a battle of wills with the demon while in the Fade consciously, or make a deal with the demon and let it in. All this, is while they're in the Fade consciously. If they go to sleep, they are in no more danger than a templar who also goes to the Fade to sleep.
And once a demon is outside the Fade, it's a danger to everyone and everything around it. Anything is literally able to get possessed at this point....I kind of want to see what a possessed boulder does. Trees become sylvans, so what would a boulder become?
But I digress.
To make the claim that all mages are dangerous is true. So too is every templar. So too is every wolf with fangs. So too is every bear with its fangs and claws. So too is every human and elf with a sword.
The human grand cleric goes to the Fade every night. The Knight-Commander goes to the Fade every night. And their dreams are viewed by spirits and demons both. Why aren't they in danger of being possessed? Mages aren't in any more danger than they are when they sleep....unless they are a somniari.
To say otherwise is to completely disregard both the lore and what Gaider said. These are the facts. Like it or not, an average mage is no more dangerous than the average templar.
If a mage summons a demon, enters the Fade consciously, or encounters a demon already in the world, they are in danger of becoming an abomination. But never any other time. If a templar is focibly pulled into the Fade by a magic ritual or encounters a demon outside the Fade, they are in just as much danger as the mage is of becoming possessed. But other than those circumstances, neither is in any danger of becoming possessed.