Hm this is a tough one.
I think when talking about Dorothy's powers we should not talk about her op dream powers when she was possessed by an elf, rather her human dream powers. Like any elf possessed where their magic power, range and force was multiplied by quite a lot, i think that should count for Dorothy as well. Maybe the force of the things in the Dream World could be as powerful as her elf counterpart, but i think its a different story with the range and the casting of the spell. I dont think the range of her spell is as big as the range she had when she was an elf.
If Fana can keep up with Noelle's speed in her Valkyrie armor, maybe she can outmaneuver Dorothy's spell casting, since it is kinda slow. One other thing is that Dorothy is kinda a glass canon. While Magna + Luck combo attack (that defeated elf Dorothy once the Dream World was broken) is not weak by any means, i think Fana's fire attack would probably have far more force since her magic power waaaaaaaay above them. Then again first she needs to get hit. If elf Dorothy's magic was way stronger, human Dorothy is a captain level combatant with experience, so hitting her would not have been as easy as hitting the elf imo.
Its really hard. Question is what can Fana do if she is trapped inside the Dream World. Probably not much, but there should also be a limit to the Dream World as well. I dont think Dorothy should be able to hold the spell longer than her elf counterpart. I presume you need huge mana reserves to sustain a small pocket dimension like the Dream World, or there is also the question if Dorothy able to hold someone in her Dream World that has magic power far above her.
Its all speculation and assumptions on my part. I'll think about it more.