I know of two references to the fabled Youkai Mountain industrial zone.
MoF prologue: "sound of distant of machines... perhaps the tengu's factories have awoken".
Soku vs. dialog, Sakuya wins Aya: "Where do you do that movable type printing? That mountain isn't that big anyway."
> The chip image
When it comes to imagery used in Touhou games, there IS a risk of overthinking things. Every piece of art ZUN and his friends did not create is a stock image pulled from some image library. So is this chip image. I believe it's here to represent "technology", that's all.
I don't see how this supports your theory.
I'd only think that if the symbol was the "symbol of spell card rules" it would appear even more so I"m not convinced. Has it appeared in more recent games?
That's just a pet theory, not a serious opinion. It hasn't been used in other places since MoF.
It's probably meant to be crosshairs.
You talk about Stage 3 and how its a call back to IN, but isn't every stage so far a call back to a previous game? Do those all have residual seasonal symbols too?
In terms of DDC's own season, the maple leaves in interface and throughout the stages should be able to establish the season as autumn.
However, you are right about the non-autumn seasonal symbols in Stage 1 and 2. The lotus is a symbol of summer, and the willow is a symbol of spring. So, Stage 3 in itself is not evidence for the game to be set in autumn, since the stages seem to be partially themed around the seasons. (Stage 1 is definitely referencing EoSD, but I'm not certain Stage 2 is about PCB.)
How does the full moon indicate that it's during the harvest moon festival, aren't there plenty of full moons during the year?
In East Asian culture, the moon is a symbol of autumn. The harvest moon festival, and the viewing of harvest moon, is attached extremely important cultural values. There are 12 full moons a year, but this one is the one that matters.
Previously I used the term "middle autumn" (the festival's Chinese name), but the festival generally falls in September, early autumn. IN is established as taking place in early autumn. Since the lotuses are still in bloom, and the willow leaves are still green, so is DDC.