Why not "truly" for באמת?
Lose or win, it is not really important. Is this incorrect?
It means the same thing idiomatically, but these are verbs whereas in the original sentence these are nouns.
Should Victory or loss, it doesn't really matter be accepted as well?
No, because the order would be wrong. הפסד is loss.
Also, doesn't matter is not the same as not important. חשוב means important.
Is defeat a proper translation of הפסד?
Maybe in this context, but I don't think it would work in any other
אוקיי, אז מה המילה הרגילה ל"defeat"בעברית?
My dictionary says it's either מפלה or תבוסה
But הפסד is what a sports team does. Sports reporters might characterize them as a תבוסה or a מפלה, but these are just stronger words (think win vs vanquish)
Whether a sports team or a presidential election, it matters.