You're making a lot of assumptions with no data to support them.
A.) You have no idea how much the levitation of the castle bleeds over to the dragon fights. Data is not available.
B.) You have no idea how "insane" (is that a measurement?) the damage is to either dragon. All we know is that it's enough to take all dragons out of the fight. Is it mostly insane? Completely insane? A tad insane? A smidge insane? My-mom-is-kleptomaniac insane? So insane it's not even insane because it's done an insane loop-back on itself?
C.) You have no idea what will or will not be affected by shape shifting. For all we know, Morrigan knows the Fifth Step Sutra of the Shapeshifting Mantras and thus can use the dragon form not only for damage but for damage mitigation of her "real" body. The data is just not there for you to make assumptions.
You and others are just arse-pulling data from nowhere to support some theory you have about the motivations of the writers. Which is silly.
you are mostly making convoluted points here:
a)The dragons are free to fly all over the skies,their flying ability is not influenced by the levitation which influenced only the rocks this is a fact not an assumption.
b)The red lyrium dragon lost half life bar,half life forces in the battle despite it was not crushed on the floor and this is a fact.
Morrigan should be dead because to all those damages she got from the opponent we have to add the clash with the floor for the gravity effect.
c)You are being ridiculous here,Shapeshifter don't heal the damages just because they returned in their original form and this is a fact they took all the damges so don't even try to use "magic reasons".
you are the one who used assumptions against facts just for being biased toward Morrigan and accuse the others who used simple logic of being silly.
As far as I know, Morrigan simply has more magic to defend and potentially heal herself while she was falling down. Maybe she reverted to a human before she landed?
In no point Morrigan performed any healing magic on herself while was in fly combat with the other dragon,even if she did that(which you have no proof of) still gravity would have killed her.
It was plot armor.