All of the example sentences have 中止 interpreted as cancelled, and I’ve never seen it used in the wild as suspension as if something is just postponed to a later time.
So why does WK have the primary definition as something that’s likely to lead to confusion if seen in the wild?
It’s also used to say, for example, that the trains have stopped running due to severe weather or something - in English, you’d say that the service has been suspended. The trains have been cancelled, yes, and that specific part of the timetable won’t be seen again, but the service has been suspended. It’ll be resumed when the weather is over.
I’m trying to find a picture of a sign at a station or something to back me up on this, but Google’s coming up with nothing for me, and while I was there in 2019 when services in Tokyo were suspended due to typhoon Hagibis, I don’t seem to have stopped to photograph any of the signage.
I thought I could use a Final Fantasy screenshot to make the point but they use 運休中:
Kenkyuusha says:
(a) suspension; (a) discontinuance; abeyance; a stoppage; an interruption; a standstill; a stay; a hold; a halt; (a) cancellation.
Clearly “abeyance” should be the primary meaning on WK.
More seriously, I intuitively find that “interruption” is the most obvious translation for the term (stopped in the middle), although of course in some cases “cancellation” or “suspension” will work better in English.
Yeah, I went rummaging through the various JR line status pages to see if any of them use it, and while there are currently several lines suspended (and which even use the word “suspended” on the English versions of the pages), the Japanese text uses words like 運休 or 取り止め or 見合わせ (which really doesn’t look like it should mean “suspension, interruption”).
Well, if you think you could swing that, you’re free to campaign for it. 
And yet…
If all you knew was WK you might ask, “Ok, so when will they set them off? An hour from now? Tomorrow?” Oh, next year? Ok.
Given this, and ALL OF WaniKani’s example sentences, I rest my case.
“This” is you cherry-picking a specific definition by explicitly searching for 足立区花火大会.
That suggests there’s an issue with WaniKani’s example sentences, which wouldn’t be the first time. Meanwhile, Jisho’s example sentences include:
I also managed to find a JR East press release that uses 中止 twice (though admittedly it also uses 運休 ten times).
But “suspension” does not imply “short term”, does it? I’m not a native English speaker but for me “suspension” is not the same as “delay” or “rescheduled” or whatever. It just leaves open the possibility that it may be temporary and could eventually be reverted, but it doesn’t have to be.
For instance:
Son of GAA boss Davy Fitzgerald given suspended sentence after stealing €35,000 from former employer
In this case “suspended” is completely open-ended.
Honestly first time ever seeing that word in my life, but I mean, Wanikani also taught me terms like “filial piety” so it’s not unprecedented to use words that are a bit more obscure.
It’s totally on me for not looking at the context sentences, but I thought it was suspension as in a car suspension. 
If it’s something scheduled that’s suspended I’d think any of the items on your list would be better (apart from abeyance).
Thanks for bringing this up! The fact that none of the patterns of use or context sentences use ‘suspension’ could definitely lead to confusion, so I’ll bring this up with the team to see if they’d like to make any changes.
Another place 中止 is used:
The turnstiles for the shinkansen have a little ticket slot on them. When the person in front of you inserts their tickets, two little metal tabs pop up to temporarily block the next person from inserting theirs until the first person is through. The tabs have 中止 written on them.
Link to a stock photo of the tab
I’m no English major, but I BELIEVE this is what Oxford would agree with, it feels to me that the default definition of suspend to imply something transient or reversible in nature. Impermanent. Consider the following:
Now, to be fair I know there are ways to use suspend to convey permanence…but I can’t really think of an example of this off the top of my head.
Keep in mind, this means little more than nothing as to the real topic at hand, as to answer what 中止 accurately conveys in Japanese. It’s just that people are correct to point out that cancel and suspend aren’t really common parlance synonyms in English.
I searched for the Adachi fireworks news article specifically because I knew it was used on their fliers that they were posting at the station that evening.
I don’t necessarily agree.
I see 中 used more often as something that is ongoing, than as middle or inside. 営業中 is ubiquitous in the wild and it doesn’t mean business inside, it means business is open, or ongoing right now.
If you are living in Japan you likely see 工事中 all the time as well.
lol, yeah, that’s always ongoing.
