Touhou 17: Explosions of Nostalgic Era

So, if I was to design Touhou 17, it would be a fucking mess
The story involves a green-haired girl falling from the sky, crash landing on Reimu's roof. Obviously worried the roof broke and she won't be able to pay for a replacement, Reimu checks to see what fucking crashed
This girl was unconscious, and after a while, awoke in the shrine, on a futon. She goes to explain who she is, and what she is looking for. Actually, it's who.
The girl was Mima, but as to why she fell on the roof? She misstepped, and fell into a hole in the ground. Some falling later, she sees the sky of all things. It's a miracle she survived the fall, really
Reimu, suspicious about the backstory, says she remembers the name, but is unsure if Mima is telling the truth. And so, she challenges Mima to a danmaku battle. Since Mima hasn't been in a game since the PC-98 era, she doesn't know what the Spell Card Rules are, and unintentionally breaks them.
This convinces Reimu that she isn't lying, because Mima hasn't been around for a really long time, since before the rules even. After the quick battle (Stage 1), the two settle back down, and Mima reveals why she was even about: apparently the shrine was uninhabited for over 20 years, with only Mima to guard it. Reimu disappeared one day, never to be seen again.
However, Reimu says that she never left the shrine, and shows Mima around, making sure to ask if things were like this in her head, and the results were an expected yes, although the decay was much worse where Mima came from.
The thing that disappointed Mima the most on the tour was the lack of Genjii, but Reimu calmly states that he rests often and isn't relatively active like he used to be, so he's off hiding somewhere safe.
This brings Reimu to her next point: what caused Mima to fall in the first place? A hole in the ground leading to the sky isn't normally possible, she reasoned, and an old enemy turned friend falling as a result of that is even weirder
There were only 2 beings she knew of that could do this, she claimed. Keine Kamishirasara, and Yukari Yakumo. They probably worked together to disrupt her day, didn't they? But why Mima of all things
Before Reimu could finish her tangent on how the older day youkai weren't as easy to resolve as the newer ones, an ominous shadow crosses the horizon.
A tank was strolling past. A tank almost covered in plants, with her Yin-Yang atop, followed closely behind by a winged eye.
While they don't seem to be inconveniencing anyone currently, they could pose a threat to the measly donations Reimu receives, and so she goes off to investigate, leaving Mima behind. (Alternatively, it is Mima who chases after the duo of strange objects, letting Reimu rest for what Mima thinks is the first time in 20 years.)
Before the mysterious duo can turn themselves to their assailant, a pod crashes down, instantly crushing whatever was inside.
The door swings open, and out pops 2 intrepid scientists, willing to go to great and often reckless lengths to study magic. They mark the Stage 2 boss, with the tank and eye forming a midboss of quick disposal.