I see that there is some effort in the research, it's all too amalgamated and muddled.
Why use the Japanese name of the flower but reference Chinese myths?
I am confused to as why you broke the myth up into three parts, even though they're all from the same Chinese myth. I also find it weird you put in Japanese terms in it, such as mixing a Shinto belief (Yomi) with a Buddhist belief (Sanzu river). (Your 40s/orthos Yomi and "jigoku no kawa" deskmat...)
Higan is a Buddhist holiday exclusively celebrated by Japanese sects during the Spring equinox (shunbun) and Autumnal equinox (shūbun), and the "Higanbana" name derives from that it blooms near the equinoctial week, rather than referencing the holiday.
Another popular myth on where the name comes from is that because it is poisonous (Lycorine, Galantamine), eating it will take you to Higan (death). (Ironically, this "flower" has been eaten, mainly its bulbs, since its toxicity can be removed with rinsing. There is also a Chinese myths as such, overcoming a harsh winter with no food with Lycoris bulbs, as they had a supply only because many thought it was truly poisonous and avoided it. This is also why it can be called 悲願花, as you would only eat it during desperate times.) As it grows wild throughout Japan, there is said to be at least 1,000 variant names for it.
Also...
Most Koreans do not know this flower as it only grows wild in very small areas. Also they call it "相思華" there and its meaning is romantic, referring to the flower always being all flower or all leaves: "The flower thinks of the leaves, and the leaves think of the flower" = "I think of only you." I am not sure where you found those other symbolisms.
btw it's 花言葉 in Japanese - 花語 is Chinese.
Japanese -- "Passion," ”Independence," "Reunion," "Resignation," "Reincarnation," "Sad Memories," "I think of only you," "I look forward to the day I see you again."
Nothing wrong about being a Chinese set, especially with GMK Shanshui sort of starting a trend. (Also sorry but your novelties scream Chinese...)
Also, I will put that GMK Hinokoku is the most fail Japanese title for a keycap set so far (5 second Google translate > "Let's go!"), so you won't top that.