Something I’ve been wondering about was if other people could still understand if you accidentally mixed up On’yomi and Kun’yomi reading in jukugo words. If you pronounced 下町 as「かちょう」 instead of「したまち」in a sentence, at a speed of about 2-4 words a second, it’s not going to throw off other people too badly, right?
I guess realistically the listener would ask to repeat the sentence and correct you, but I’m still curious.
No, they wouldn’t be able to understand you in this case or a vast majority of other cases. Depending on your pitch they might think you’re trying to say 課長 at best. It’s no different than how if you said a word with a completely different pronunciation in English people wouldn’t understand.
Indeed, “Shitamachi” is term that describes areas of Tokyo like Yanaka and Nezu. “Kachou” is… not. You’re gonna get some blank looks if you say “I’m going to visit Yanaka to see the kachou”.
I think if I say I am going to visit the Netherlands in New York, you wouldn’t understand me either.
I meant “downtown” but I read down as nether and town as land.
Think of kanji as being no different than english letters (except there’s way more of them). If I told you, “I broke the kip her” would you know what I was talking about? Kip her? What’s a kip her? Turns out I meant “cipher”, see ‘c’ can be read as ‘k’ or ‘s’ and ‘ph’ can be read as ‘f’ or ‘p h’ so it shouldn’t matter right? But the word comes before the spelling.
下 is sometimes read as か and sometimes read as した, but the word is したまち and it’s spelled 下町.
Mind you, there are some words which can take either reading, and even some words for which the reading is fairly fluid, by public consensus. For example, the word 私立, しりつ, can also be read as わたくしりつ, because otherwise it’s a perfect homophone for its antonym, 市立.
大人 is read as おとな most of the time, but when it’s grouped with 中人 and 小人 on a list of ticket prices (for example, this page on the Tokyo Disney Resort website, about halfway down the page), the reading is だいにん instead. (You should still use おとな in general conversation, though - I’m just providing this example so you don’t get too complacent. 
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For anyone interested: the reading くだ of 下 is from ください. You also see ください written as 下さい sometimes. The best mnemonics are the authentic Japanese mnemonics. You will never forget this reading again.
Mind telling us what this unforgettable mnemonic is?
I guess the assumption is that you would be familiar with ください already and you could backtrack the 下 reading from that.
I actually do that with 握る using お握り, 好む with お好み焼き and 任す with お任せ. Yes, I like food…
So, not a mnemonic but another variation on: “If you know the readings of your kanji you’ll know how to read this as well.” 
I find those only work for me when it’s a standard ‘On+ON’ or ‘Kun (+ kana)’ combo and the kanji only has one reading. Not the mess that 人生上下 etc. are.
The idea is more that you’re likely to know those readings before studying the kanji at all, and you can tack the kanji on top of that knowledge, so to speak.
Yeah those are always tricky but at least 下さい has consistent okurigana which makes it easy to find the correct reading. した・もと・しも are much trickier of course.
I feel like “from” isn’t the right word to use here, but it’s actually… from 下す. The 下さい form is an older conjugation of 下す that’s been codified as its own word over time.
You’ll also frequently enounter 下り (the stem form of 下る) when you’re looking at train timetables - 下り means, by convention, the direction away from Tokyo. The antonym is 上り, read as のぼり.
Here is an example, plus a screenshot of the relevant part, in case JR Central overhauls their website, again.
お好み焼き
I thought I corrected the okurigana but then I just dropped it completely (お好み焼 does have an entry on JMdict though, so I win).
But generally on the discussion of mnemonic or no mnemonic, in the end it’s very subjective and depends on the situation. What I wouldn’t recommend however is to learn くだ on its own as a reading for 下 because these truncated readings are pretty meaningless. Learn 下る, 下す or even 下さい instead.
I point it out because unfortunately WaniKani does that in some parts (teaching truncated kun readings for a kanji) and I always find it silly.
It’s my experience, but I learned ください / くださる from grammar, and getting a little of blurry visual memory from 下さい in print. So it’s pretty clear it’s gotta be くださ… or shorter.
I learned 下す way afterwards, but this one is in several of memorable phrases.
For this, I learned 登る from a textbook first. So it’s a good example of alternative Kanji, 上る.
I would insist on this, even when you don’t know any of associated vocabularies yet. Often, you might be able to remember Kun’yomi and the sound of the vocab, even if you can’t recall well what the vocab really means.
But what Kanji really means is often easier to infer from the shaped of the Kanji and radicals. (And so, relearn the vocab meaning after having remembered the Kanji.)
I did the same, and Wanikani also teaches 昇る.
