Warning: My Japanese kinda sucks. You can take my comment with a grain of salt...

At some point in learning Japanese you may have come across a sentence like this one: 象は鼻が長い。("As for elephant, nose is long" = "Elephants have long trunks"). 象 is the topic: it provides a context for the rest of the sentence; it tells us what the sentence is about. What is it telling us about this topic? It says their trunks are long (鼻が長い).

Note, however, the particles don't have to be "...は...が". I'm pretty sure all the following are grammatically correct (correct me if I'm wrong or mis-understanding them):

象は鼻が長い - Elephants have long trunks. / The elephant has a long trunk.

象は鼻は長い - The elephant has a LONG trunk [...but a SHORT tail, etc.]

象が鼻が長い - The elephant has (is the one with) a long nose. / Elephants have (are the ones with) long noses.

象の鼻は長い - Elephant trunks are long. / The Elephant's trunk is long.

象の鼻が長い - [It is] elephant trunks [that] are long. (Maybe "[Wow!] That elephant trunk is long!" is possible too?)

Note: You could also scramble some of these too ("鼻は、象が長い"), but that's unlikely. Also, you can't ever do "...が...は" because that doesn't really make any sense: "The elephant is the one that... Their noses? Yeah, they're long." "The elephant is the one that... Guess what? They have long noses.". Other usages of は I won't be discussing are asking a question about something ("あの人は誰ですか？") and when は is attached to something that's not the subject.

In general, The topic is the old information (context) that new information is being provided too. The topic can also be thought of as being an instance of something that the predicate applies to: 魚は泳ぐ "Fish swim. (Fish belong to the set of things-that-swim.)".

Another usage of は is indicating contrast. In "...は...は" sentences the contrast is obligatory for the 2nd は ("...but a SHORT tail."). This contrastive は is also used when the sentence is negative: ペンはない。"There is no pen". Outside of these situations, は might still be contrastive if context gives it a reason to be 魚は泳ぐ ("Fish swim [...but birds don't]")

So why is it 況して、向こう*は*主将は不在? Note that "象は鼻が長い" and "象の鼻[は/が]長い" are logically equivalent. It's the same for 向こうは主将は不在 and 向こうの主将は不在. The difference is what the topic is--that is, how the information is organized.

So why is it not 況して、向こうは主将*が*不在? I'm not entirely sure. I think it's because "不在" (abscence =lack of presence) is a negative which inherently creates a sense of contrast. If you used が, the sentence would sound like "Not to mention, on the other team it was their captain that was abscent." instead.