Apparently, I am on a roll with my research as I have just accidently found all of Distant Union works in one fell swoop.
This is the link: https://doublepuma.com/doujin/
As for how I came across this link... I started by putting random words into Google like site:tumblr.com "touhou" pdf. I noticed the askudonge tumblr account and clicked on the link since its Google description said "But it's now a free pdf!". It turns out this is a pdf of Raiders of the Lost Ask Box, and I was about to click off of the pdf since that fan book was already uploaded on Lina's Art Gallery.
But then I saw the url: https://doublepuma.com/doujin/raiders.pdf
I asked myself: How could doublepuma.com still exist? I thought that website was unavailable?
Of course, I now know that I was confusing doublepuma.com with doublepuma.bigcartel.com. But I didn't fix my mistake there as I soon saw that the url looked like a web directory, so I decided to up in the directory by removing the pdf part from the url.
Fun Fact!
Did you know that my visit was the first Internet Archive crawl into the page? I would know since I run the Internet Archive extension to save pages that don't have an archive for this exact reason. This led me to believe that no one from Distant Union ever actually posted the link, since the front page for doublepuma.com just leads links of Lina Ciari's works, that haven't been updated since 2020. No one would naturally find doublepuma.com/doujin without someone linking them naturally to this part of the website. So I did another google search for "doublepuma.com" to check on how people were posting links to this website.
Did you know that that Plus on Block, another Touhou work by Lina Ciari, was featured on the 26th broadcast of Touhou Station? Found that one because it was submitted with the link: https://doublepuma.com/doujin/pobdigital.pdf
Also, a moral of this search is to always use a Nitter instance as a part of the internet research process. It turns out that Lina Ciari already posted a pdf link to The Gensokyo Menagerie Vol. 1 on their Twitter account this entire time. It was the only Distant Union doujinshi that got posted publicly so this search was not a total waste, but it is a reminder to always look into Twitter as people post on that social platform far more than any where else.