The author is using a curt style of writing because he's talking about simple things. You could use both "for comparison" and "by comparison". "for comparison" is a bit like "if I may show you comparative data", similar to "for example". "by comparison" means "to illustrate the point by using a comparison". The two are not much different in the end. "opened to (an amount)" can be taken as "opened to the tune of (an amount)", where "to the tune of ..." means "to the extent/scale/amount of (an amount)". So "opened to (an amount)" means "opened grossing/making (an amount)". "en route" means "on the way to" (en = French for in/on, route = way). Your sentence can be paraphrased as "The Force Awakens opened grossing(making) $52.3 million and is on its way to making an accumulated total of $124.1 million in China ...".

This excerpt isn't very well written. I agree with you. "By comparison" is more appropriate here because it's comparing the income of two movies. "For comparison" is used to introduce the subject matter. For example, I could say, "For a comparison of top grossing movies, let's consider these two examples..." "Opened" refers to the date the show/movie started. "En route" is a French expression meaning "on the way to". (Wikipedia says that 29% of English comes from French.) Yes, it's used correctly in the sentence, though it's strange that the writer knows the box office total of each movie... unless this article was written after the fact.