Meaning – Does ‘Upset Victory’ Mean ‘A Victory That People Are Not Happy About’?

I think both of your suggestions sound a little off. I'd suggest

Not likely=

(1) I probably haven't finished my homework.

(2) I don't think so. (with a heavy emphasis on think – to indicate sarcasm, as mentioned in The Photon's answer)

However, I must ask: Did this question come straight from a test? Or did you make it up, or alter it somehow?

It seems unlikely that the phrase "not likely" would be used in the first person, not if we are talking about something that isn't in the future. The word "likely" is used to express a high probability. Therefore, if you ask me whether or not I've finished my homework, either I have finished it, or I haven't – there's no "likely" involved, because there's no uncertainty involved.

I could see the phrase not likely being used in this context (future tense):

Do you think you'll have your homework finished by 10 o'clock?
Not likely. (Meaning: "I probably won't.")

or this context (third person):

Do you think Ariana has finished her homework?
Not likely. (Meaning: "I doubt that she has.")

However, it's very hard to imagine me using "Not likely" as an answer to the question, "Have you finished your homework?" Then again, perhaps that's the point of the question; if it's meant to be an incorrect answer, maybe it's not supposed to sound interpretable.

This is slightly more complicated than LawrenceC makes it out to be.

Symbolic can have two connotations: one positive, one negative.

Positive: this is best portrayed in your very first sentence, but also in the third and the fourth.
The expression symbolic victory here means that this is an important victory, as it shows the people (most often in battles, be it wars or even fights against the government) that the fight isn't over.

Since it is possible to win one battle, like the symbolic victory demonstrated it, there's no reason to surrender the war. Thus, the victory here is a symbol of hope for the people.

Negative: this is the connotation LawrenceC talked about, where the victory is only symbolic and won't actually change anything, as you can see in your second sentence.

Both these connotations have stemed from the original meaning of serving as a symbol of something, but have grown quite different in meaning depending on the context.