[...] but I dunno, it seems like we’ve seen this somewhere before. Somewhere maybe around, say, Episode 3? Given that it was only two weeks ago, isn’t it a little soon to be pilfering from your own [...]

As for mal-tu, that’s a bit different from the polite/familiar (joen-dae/banmal) thing. Mal-tu means more of your speech patterns and behavior (i.e., Anna Jo in Fantasy Couple is described as having a very hard mal-tu, because she uses a sharp tone and harsh language). As for speaking to one younger than you, if you’re talking to a stranger, you should still use polite speech. Speaking banmal to anyone (who’s an adult; children are different) before you know them well is considered very rude. For instance, Yoo Hee starts out using polite speech, but switches to banmal with Mu-ryong very quickly, which he finds inappropriate.

But, once you get to know each other better, the senior person in the relationship MAY drop the politeness and use banmal… but it depends on the circumstances. If you’re conducting business, for instance, stick to polite speech no matter who’s in the superior position. It’s all very complicated.