Have you ever looked into the possibility of scotopic sensitivity, or visual tracking and convergence issues? These can all cause inconsistent performance on reading tasks, varying with fatigue, lighting conditions, print size and spacing, contrast, and other conditions.
Are the accommodations helping her performance? If they are, then you may want to look carefully at what other issues that are not dyslexia (problems with executive functioning, attention, visual processing, motor skills...anything at all) would also be helped by such accommodations, and make that the short list of ideas to investigate further.
FWIW, anxiety could certainly contribute to guessing, paradoxically out of fear of getting the word wrong in front of whoever she is reading to. She encounters an unfamiliar word, the anxiety goes up, the ability to access the information and process she needs to attack the word goes away, and guessing may be all she can manage.
Good luck with getting it all figured out. It's a grueling process.
