I’d say the primary thing to mark as spoiler would be any English translations.
I’m starting with mention of what the の is doing, then at the end I’ll address the な. The following is going to be a bit to take in, but don’t worry about it! This grammar will come up a lot, so I’m sure it will be asked about again.
The heart of this sentence is:
「なんで うんち だ」
“Noun sentences”, that is sentences that say “[noun] is a [noun]” end in だ. (There are exceptions, but we’ll see some of those later.)
Consider if you are saying the English sentence, “The paint is wet.” That’s a simple “[noun] is [noun]” sentence.
But consider if instead someone asked you why there is a “don’t touch” sign on your fence. You might replace “It’s because the paint is wet.” Basically, it’s taking the sentence and adding “It’s because” in front of it.
In Japanese, this same concept is done by taking the sentence, and adding “のだ” to the end of it. (This is the usage @foxabell mentioned.)
Now, for the answer to the specific question: Why is there a な before the の?
When adding のだ to the end of a sentence that ends in だ already, you don’t say 「…だ のだ」. Instead, the first だ changes into な, and it reads as 「…な のだ」
This is a “[noun] is doing [verb]” sentence, with the verb being いる. The verb いる is used to state that the sentence’s subject “exists”. (The subject is the noun marked with the particle が.)
Japanese verbs don’t have tense like in English (past, present, future). Instead, they are either an incomplete action or a completed action. Completed actions have た added to them (in this case いる, changing it to いた).
If いる means “exists”, then いた means “existed”.
But that sounds a bit weird in English, doesn’t it? “A panda-like cat existed.” However, we can word it a little differently for something that sounds more reasonable to an English speaker: “There was a panda-like cat.” Same thing, worded differently.
だ is used at the end of “[noun] is [noun]” sentences.
When used at the start of a sentence, as I understand it, it’s essentially referring to what came it. (Really bad explanation on my part, I know.)
The って is used for (indirectly) quoting someone.
Put these together, and you have だ (referring back to what was said) and って as an informal quote, and it’s kind of like in English if we say, “You say that, but…”
In various contexts, this だって may translate as “but” or “because”. (This is due to an imbalance in precise meanings of words between English and Japanese.)
どんな requires a noun, and were we have の filling in for the noun.
Consider in English if someone asked you to hand them a box of cereal. You can say “Which box of cereal?” but you’d probably just ask “Which one?” This “one” is standing in for the actual noun.
Likewise, this の is standing in for an actual noun. What noun is it standing in for? This is where context comes into play, which can take some time to build up. In this case, it’s referring to the panda-like cat.
Something like: “What kind (of panda-like cat) was it? Try drawing it.”
Your translation didn’t quite make it through, so I can’t quite comment on that. But, I can break the sentence down a little, and if you have a question on any specific part, just let me know!
それ＋より = that + more than. This wording is common, with the meaning of “more than that” or “rather than that”.
アジト = This refers to their secret hideout, the little clubhouse they’re in.
に = A particle that identified a noun as a destination. This is marking アジト as the destination of an action.
こんな = Like the word “this” in English as in “This cereal is mine.” It requires a noun follow after it.
の = Fills in for a noun. Like in “This one is mine.” It’s substituting in place of the actual noun.
落ちてた = The verb 落ちる (to drop) in the joining form 落ちて, which joins to いる. This gives the meaning “dropping”. And then add た to mark the action as completed, “having been dropped”. The い is often dropped in such casual speech as we have here.
All together: “Rather than that, such a thing as this had been dropped in our hideout.”
This is Yui (the girl who run into the hideout) speaking to Sacchan (the one with the sproingy glasses). She’s saying:
もう, but exaggerated a bit that it comes out as もお = “geez” or “come on”.
マジメ＋に = serious + the destination particle = do (verb) toward serious = do (verb) seriously.
聞いて = short for 聞いてください. 聞く means “to listen”. In the joining form 聞いて, joining ください “please (do for me)” results in a meaning of “please listen (to me)”.
よ = give emphasis
Together: “Geez, listen to me seriously!”