No Disability
In order to determine what's going on that Hisao doesn't know, we need to take what we have, and see if we can get something internally consistent. That means answering the following two questions:
Why did Misha choose to come to Yamaku? She initially didn't want to come, and choosing to come meant turning her rudimentary sign language knowledge into a career in order to pay tuition. This is not a "path of least resistance" situation.
Why does Misha not miss anyone from her previous school? Misha seems to generally like people and be friendly toward others. Not missing anyone is indicative of a problem.
Being bullied for unspecified reasons answers the second question, but there's no reason why she'd want to transfer to Yamaku rather than "some other school" - Yamaku is a special school, and expensive.
Being bullied for being a lesbian only leads to the question "Is there some stereotype about cripples that they don't mind homosexuality?"
Being bullied for being "fat" almost answers both questions, but we still have the question of "Why Yamaku?" Misha didn't know the specifics of the cultural festival, and she did care about the specifics before Shizune spoke up. Therefore, Misha wasn't local. Fat Misha could conceivable, but it's a bit of a stretch. Also, looking at the height and weight, that gives her a BMI of 22.8 - squarely in the "normal" category, which many nutritionists actually consider slightly underweight. Her measurements also put her as nearly hourglass, if not quite the "magic" proportions of 3/2/3.
I guess you can argue that her needing the sign language scholarship to afford tuition was a lie, but that seems extremely irrational...
Inner Ear
We get her "dizzy on stairs" line when Misha and Shizune are trying to fast-talk Hisao into doing some work. Shizune's excuse is that Nomiya skeeves her out. Misha's is even weaker. This is a school for people with disabilities. As Emi's arc points out, this includes people in wheelchairs. Yamaku is equipped with elevators. On top of that, we see many indications of Misha using stairs, even while encumbered.
As a counterpoint, she confirms dizziness on stairs without the slightest hesitation when Hisao asks her about it several months later.
On top of that, she has her vocal idiosyncracies.
Dizziness (on stairs) is a common symptom. To tie it to inner ear issues is like deciding that a cough means you have Tuberculosis - it may be accurate, but it's a pretty huge leap on its own. Even taking into account her loudness and tonal speech, it still feels like a bit of a stretch, especially as she indicates no difficulty hearing others. Later revelations also seem to imply that she does have volume control, that she simply chooses not to use. Finally, we tie back to the "No Disability" arguments. Inner ear issues are unlikely to alienate peers, and the issues stemming from inner ear problems seem unlikely to justify the proactive steps needed for the Sign Language scholarship.
Besides, Takashi Maeda already has tinnitus. I think there's something in the Yamaku charter against having two people with similar disabilities in the same classroom.
