The sentence "Just the boy really feels something, the girl hates him." has two problems.
The first, as Barrie says, is the comma splice between two independent clauses.
The second is the contradiction. Both the girl and the boy feel something. Presumably, however, the boy loves the girl but the girl hates the boy. Hate is a feeling.
Neither only nor just is appropriate here.
The relationship between the two adolescents is {unbalanced / unequal / antagonistic / incongruous / inconsonant}. The boy loves the girl, but she hates him.
A fact does, in fact, have to be the truth at the time you're using the word.
By 'truth', I mean something you believe to be true (due to any of several possible reasons).¹
Consider: "The number of planets in the solar system is eight."
A few years ago, this was not a fact. It is now. (Just an example, don't attack the example.)
[EDIT: Before 1917, people thought it to be a fact that the atom was the smallest particle of matter. Today, it is a fact that it isn't, and we were wrong in thinking it was... I'm including this example to show that in light of the new fact, what we thought was a fact previously, can be rendered untrue for that time too; something @Jay pointed out my previous example didn't specify]
When a jury convicts a man, it's a fact that he's guilty. If he's later acquitted, it's a fact that he is 'not guilty'. (As far as the public is concerned. Individuals who actually saw the crime might know, for a fact, whether it's true or not)
What I'm trying to say is that the word fact is used for what you know (or sincerely believe) to be the truth or what is widely believed to be the truth at the time of speaking. Facts are subject to change.
Something is not a fact if you know/believe it to be untrue or if it can be easily be shown to not be widely believed at the time.
The two can contradict. That's when myth comes into play.
Tom: "Interesting fact: you can see the Great Wall of China from space." (Widely believed).
Neil: "As a matter of fact, you can't. That's a myth." (I went to space. I know better.)
¹ Please note that I'm talking exclusively about the cases where you use the word fact. In those cases, I infer you strongly believe it to be true.
Best Answer
There is no way to completely remove ambiguity from English and anyway, we wouldn't want to do that as different interpretations of language can be interesting, humorous, challenging. Regarding your question I would interpret this version (I have fixed the dangling modifier):
As meaning: "all stalls in the market are operating between 9am and 5pm but some stalls may also operate outside of these hours".
To state the opening times and a busy period I might say: