Is Ichijiku no Hana a rightful part of KSK?
 

Multiple choices allowed, because it seems like a reasonable stance to hold.

Its of the KZK-era, but I really wouldn't say its a KZK song. Like Mellow, So Cold, Gamble are to SS (though I realize their release was a different matter; there wasnt a "ZCS era" though that post-SS time was its own neat island)

Tangentially, I wouldn't say the Mayonaka single was of the KZK era or the SS era.

Hm...
I would say no

Because Ichijuku no Hana seems more in line with the career retrospective that was Ringo no Uta than anything else (especially with that PV)

So, breaking it up into eras:
MM
SS/ZCS
Mayonaka
KZK
Ringo no Uta (+Ichijuku no Hana)

That make sense?

I wouldnt go so far as to say Ringo no Uta (/ichijiku) is its own era as Mayonaka (/UM) since they were both likely written/recorded with, or very near, the other KZK songs.

KZK song, no (as in, i wouldnt tag it on at the end of the record, lp be damned) but certainly in that creative sprint. (i guess La Salle de Bain falls here too?)

oh wow I'm the first yes vote

So I've listened to all the tracks as they would have been arranged on the vinyl, and it doesn't seem out of place at all. Kinda wish it had been released on the CD to soften the blow of putting Souretsu on the tail end of the album, even if it breaks symmetry.

Even if it wasn't just tacked onto the end it would probably still fit and deserves its spot on KZK.

Yes and no. For most people a no. But if you had heard the vinyl first for whatever reason, that is the version you will remember the most.

The same question could be asked about whether Ariamaru Tomi belongs with SG etc.

total yes, I always thought it should've been the official ending of KZK, but not part of the inner symmetry, more like an epilogue to the whole album. I think Ariamaru also should've been on SG together with it's b-side

How on earth could anything be after "Souretsu"? (read as: should)
Ichijiku is in my top 3 Shiina songs ever, and no way do I feel it has a place on KZK. I think its not-even-a-single isolation makes it even more fun.

if Souretsu is the end of the movie then Ichijiku is the closing credits song, that's how I see it

Unlike HF, though, KZK is not designed to be like a movie. It's a soundscape, so having Souretsu last makes perfect sense. Souretsu says "THIS ALBUM IS OVER".
If you put Ichijuku after Souretsu, then Ichijuku basically says "NOW IS THE TIME TO THINK ABOUT WHAT YOU JUST HEARD". It's a breather but not entirely necessary. I feel like it softens the blow of Souretsu, and if you need sometimes after Souretsu to soften the blow then you COMPLETELY missed the point of it.