I would agree with sanne. If she can correct misspellings and appears to have challenges in self-monitoring/self-editing, including in other mechanics areas, I would suspect that this is more of a function of ADHD-related self-monitoring than of actual encoding skills. Another area worth thinking about is automaticity skills. You report a number of behaviors related to automaticity deficits, which may be related to ADHD or to other learning differences, such as dysgraphia: handwriting, spelling high-frequency words, mechanics.
In terms of trying to isolate spelling from other factors (such as handwriting, organization, idea generation), try having her spell words out loud. That reduces the impact of handwriting. See if she spells better when copying, which removes the cognitive portion of composition. Download some free graphic organizers and try them with paragraph-level writing, which supports challenges with organization, and note changes in the quality of her spelling, mechanics, and composition. Give her a sentence starter, or do a verbal pre-writing exercise, and see if having a jump-start on idea generation and initiation helps.
All of these strategies reduce some cognitive load from writing, but from different aspects. You may be able to tease out which supports help the most, and in what way. This can give you some idea of where the actual area of need lies, as well as which strategies for supporting her classroom success in written language are likely to be most effective.
BTW, how is she with other automatic skills, such as math facts, tying shoelaces, jumping rope, riding a bicycle?
