Thing is, her mana never goes anywhere. She can control it, sure, but it's not discharged or spent in any way. Likewise, if the body can regulate mana the same way that people can breathe, then it stands to reason that her disease would not be an issue in the first place. She would have a lot of it, sure, but the body would just discharge it like normal. Unless she can't discharge it like normal, but then we're back to the point where she can't really 'learn' to discharge it the same way you learn to breathe harder when you're out of breath.
Ultimately the limitation that the work has provided is that mana doesn't burn out just because she wants to. It (maybe) drained out when she was exposed to the Trombay, but there isn't concrete evidence to that.
And, of course, there is the language barrier. Within the bounds of this story, magic is not common knowledge. Education is not common, and reading/writing is something that requires effort to learn. So far we haven't seen a single instance where a person can use magic, and it's fair to say that the usage of magic by a person is, therefore, something that requires constant training and learning and therefore has very strong social connotations to its use.
TL
R Red Riding Hood. You get to fail once.
(Also Main's like 7. If she were older and had access to more resources then yeah, she would be in a better situation. But she's 7 and can't walk from A to B without passing out)