I'm also going to say that "spirit" in the sense of "spiritual energy" ("spiritual energy" or "chi" may or may not include "temperaments" as a factor of it), and "spirit" in the sense of "a soul" aren't exactly the same thing, even in Touhou, and that we shouldn't unnecessarily equivocate them. Spiritual energy, souls, and even phantoms, as such, are clearly distinguished from each other via their functions. We've never heard of spiritual energy attacks being described as having the kind of auto-kill properties touching a phantom does, for example.
Well first things first, phantoms don't have death hax, you're thinking of ghosts. Touhou does differentiate between the different kinds of spirits. Secondly, assuming that spirit power, spirit, or spiritual energy are different things is just silly. In HM, for example,
the spirit bar is just called spirit, even though it functions identically to the bars present in other games
that are referred to as spirit power. We have reason to equalize these things, because both the games and the characters themselves don't treat them with any meaningful distinction.
I'd argue that the overwhelming majority of this "Spirit Sign" stuff falls way more into the category of "gameplay mechanics" (a la rings in Sonic games, Super Mushrooms in Mario games, etc) than anything having to do with an "in-universe" function, especially given how they're claimed to work in ways that are completely incoherent with their stated function (e.g. taking soul stuff from explicitly soulless beings). To those who would argue otherwise, I would like to pose two questions: has there been literally any time a Touhou character has extracted "spirit sign" power ups, or anything similar from another character, outside of gameplay? Secondly, has anyone in Touhou lore or canon ever commented on how the 2hus in a given fight are extracting spirit/soul stuff from each other with just basic attacks?
I don't believe for a second that these topics would just be passed over if they weren't simply gameplay mechanics, but things that actually occur in-universe.
We've been over the soulhaxing soulless beings stuff, it's no different than similar showings from Yukari. But honestly that's the most controversial part of this thread, so I can't really blame you for disagreeing. However, we cannot just reject how it works in literally hundreds of instances just because it happens to work weirdly in 2. It's not 'completely incoherent' when it consistently works the same 99.9% of the time.
The 'gameplay only' argument is somewhat absurd too; As mentioned before,
Sanae's comments in UFO confirm that spirit power is a real thing the characters are aware of; They just consider it so mundane that they don't bother bringing it up most of the time. Additionally, the entire plot of SWR hinges on characters noticing that everybody's spirit is being absorbed, and they consider this a major incident. So the existence of spirit power as an in-verse thing is all but confirmed, as well as the acknowledgement of its absorption.
But most importantly, do we even need in-universe confirmation? Like, take 17.5 for instance.
When you graze bullets, they turn into water, which I plan on adding to the profiles as transmutation. Would you reject that on the basis of "the characters don't mention it, so it didn't happen"? We have an explanation for how it works, and we see how it works in action. That's good enough for most people. But spirit does at least have the distinction of being an in-universe power source that characters make note of from time to time.
Honestly, 17.5 is hardly the only example, even currently accepted stuff like player system cards have the same issue as what you're describing, and I guarantee I could find more examples.
also using Sonic's rings and Mario's mushrooms as examples of game mechanics isn't a good idea when I think both those verses do actually acknowledge both those things in-universe quite frequently lmao