However, since I've concluded earlier in the thread that the spell card rules really involve magical protection for the duelists, the problem becomes why is Mokou not protected from fatal wounds. Perhaps the protection is a smart system that automatically protects against severe damage, and as Mokou can't die, no damage is too severe for her to trigger the protection? This seems a bit too contrived for a theory without sufficient canon support.
If we are to assume Spell Card Rules provide some sort of protection, which with the evidence you supplied, suggests there is that possibility, then, we can assume fighters are more or less safe during Spell Card Rules.
However, it is too quick to simply assume Spell Card Rules provide absolute protection. In the Spell Card Rules itself, it mentions about not going all out. I would assume anyone relatively strong can break that safety net that Spell Card Rules does provide. I think of Spell Card rules as wearing protective gear, it won't save the other person if you are trying to kill them, but it will save them if you are competing.
I always thought that the deal with Mokou was that her body is SO malnourished and broken from her complete lack of care with herself that even the normally non-lethal danmaku is enough to kill her.
While we know that she doesn't care about her health, we also know that she is more than healthy enough for any kind of physical activity as seen by Inaba of the Earth and Inaba of the Moon.
It's just a question of how serious they were about that, since ZUN stated (and it's pretty obvious when you look at things like the Sakuya case mentioned above) that they typically aren't REALLY serious when they threaten to kill each other.
I agree, their seriousness in pre-battle banter is usually suspect. They usually just trash talk during their pre-battle banter dialogues. Their post battle dialogue seems to imply that they are using attacks that could be fatal. As seen in this
post battle dialogue:
Yukari: Wow, look, she's still alive. She really can't die, huh?
Reimu: I'm seriously surprised. It looked like she was about to die a bunch of times.
Mokou: Ow, ow! I won't die, but it still hurts~. Marisa's Afterword on her Grimoire gives a somewhat detailed explanation about Spell Card rules. This quote should explain the basics of spell card rules and how it should be used:
Like "Danmaku Bounded Field" and "Imperishable Shooting", most theatrical types tend to mirror the ability of the user so it's fun to watch. These kinds of danmaku exist for no reason other than the basic idea behind the spellcard rules. If the purpose was to annihilate the enemy, theatrical danmaku are complete nonsense. I'd say it applies more to high-leveled demons, but for lower-leveled ones, like Mystia and Wriggle, I don't think they can survive, like, having their hearts torn into pieces or blown up by bombs or nukes.
Wriggle is not a very good example, as she did beat up Reisen. But as some others have said: Even weak Youkai are more or less very hard to be hurt by physical damage. If the damage isn't spiritual in nature, it will likely not work. So, the use of stuff like guns, swords will likely do little without some sort of enchantment.
