Compared to Corypheus and FLemeth,Morrigan does not possess any foreshadowing ability that explain through magical means her survival against the red lyrium dragon,nor the physical condition of the incident explained why she survived since in the same scenario the other dragon the Mythal guardian is dead..
By your own convoluted logic Corypanties should never have been able to resurrect himself (which, by the way, is never fully explained and is unique to his character) because it wasn't "foreshadowed". Remember, he was introduced in Dragon Age 2. The only thing that was foreshadowed about Mr. Cory was he was so powerful the Wardens had to imprison him. That could be for a multitude of reasons.
Sorry, that doesn't work. It's perfectly legit for a writer to introduce a new character or new ability without any foreshadowing.
Not that the Morrigan situation fits anyway. Nobody knows, nor has it been explained, what Morrigan has learned from the Well. The story doesn't explain everything the Well teaches the Inquisitor. The story doesn't explain any details about the Mythal dragon, or Morrigan in dragon form. All unique situations. After all, it isn't every day somebody drinks from an ancient elve's trove of wisdom passed on through multitudes of other elves in some weird magical soup. It certainly isn't every day that someone turns into a Well-induced-dragon, nor is it every day that Morrigan turns into a Well-induced-dragon. It's a unique situation that isn't explained. No magical spells are described, nor are their limits, varieties, flavors, moods.
Morrigan herself is a unique mage. We don't know what she's learned from Flemeth. We have hints of some of what she's learned, but not everything. Then Morrigan traipses off by herself and learns a bunch more while exploring and studying ancient lore. We don't know most of the details about that either. That's what makes her dangerous and that's what makes her interesting. We don't know any details about Morrigans unique knowledge from her travels and from the Well and what affect that knowledge may have on her shape-shifted dragon form. Obviously, it was a powerful enough spell to let her survive a fight with another unique dragon, the Corypants dragon.
It's also what makes her character flexible enough to go through weird and unique situations that the story explores.
Then we have Corypants, who's magical powers come completely out of left field. Both in Dragon Age 2 (watch that fight) and Dragon Age Inquisition. Solas hints at what Corypants may be doing. But nobody knows how. All you've explained is what Cory did. Not how he did it, nor what all the permutations may be, or the limitations, side effects, range limits... you have absolutely no data at all to explain what the "Hey let's levitate a castle" spell does. All you see is two dragons coping with the situation as they try to fight. The game simply doesn't tell you anything about what the spell may or may not do to them, let alone what a plummeting dragon landing on a "floating" platform will or will not do to three completely unique dragons.
The Mythal dragon is not the same as the Cory dragon or the Morrigan dragon. All have three unique sources and have three unique outcomes from their fight in the unique battlefield. We don't know how strong each's armor is, is the Cory dragon depleted uranium grade and Morrigan is aluminium grade and she's just really fast? We don't know if the Mythal dragon just came from having lunch, had a cramp during the fight and "died" because it had trouble digesting a goat bone.
Obviously, there are enough unknowns to not leap to another assumption, that Morrigan is some writers pet character. I'd consider myself rude if I made that assumption about some author I didn't know.