Random ideas topic
We really really can, since he was unconscious when it happened. I could just as easily assume any part of his body or clothes would work (since both are spiritual particles created of his soul), or that the halo forming by his power while removing the previously created hair simply looks like that. It's not like the activation itself is necessarily either the halo or the hair, we don't actually know what the power or instigator is in this equation is at all. We have absolutely nothing on this technique or ability at all.
Again, magical fairies, shits and giggles.
No, that's the opposite of how it works.
If someone showcases a very deliberate pattern to do something like a technique, it's always going to have to be assumed that any attempts to do the same thing again requires you to take the same steps to recreate it until the author proves otherwise later down the line.
One example of this in action is the assumption that all Ceros have to be charged, meaning you can't just instantly fire it whenever you want (Unless the person in question, Ie: Starrk, proves otherwise). More over every cero ever fired has always been locked in a single direction once it has started charging, meaning that's likely a facet of it too. We can't assume beyond this without someone showing us otherwise, and moreover proving it's not something they specialize in (Like Starrk) and is actually a general technique that can be done in any given situation, but just hasn't been show until now.
That's just common sense. It helps to rectify the simple logic behind why people do such arbitrary things to utilize something, like in this case why doesn't everyone fire ceros instantly or change its direction while charging since that would be obscenely useful.
To use a more general example of this concept in work, the initial showcasing of Byakuya's bankai is activated via dropping his sealed sword into the ground to summon the wall of blades leading to the release of his sword's bankai state. From the procedure there you have to assume that he cannot activate his bankai without doing that same procedure, meaning it can't be done without his sword, while moving, or in shikai until he proves otherwise in a subsequent showcase of the technique.
Recently Byakuya has indeed proven that he can summon his bankai while in motion, against Yammi, so that's another facet we can put on the table, because he has
proven he can do it. More recently, it has been showcased that he literally cannot summon his bankai without all the pieces of his sword, which solidifies one of the other assumptions already in place around his bankai, and exposes a rather fatal weakness that can be exploited with the right ability.
Mind you there's a bit of common sense logic applied to this way of approaching things too, meaning you can't blindly assume utterly arbitrary limitations that have no basis, like say since Byakuya's never shown using his bankai at night he can't or whatever. You do however have to take the deliberate things at face value.
Sometimes admittedly it's hard to know what is deliberate or not in certain situations admittedly, which is where this kind of logic starts to get very murky. Gin's bankai for example is somewhat of a confusing mess because there's a sound argument to be made that the poison doesn't activate without him uttering the kill command and touching the wound in question.
Now despite the logic leap of why poison would even need a goddamn command to activate, it's VERY hard to argue against it with the fact that the text bubble with the command in question is VERY deliberately different with a very special kind of quote text around the command itself. That alone ALWAYS signifies some kind of importance in the words beyond just 'dramatic effect' to say the least.
Nailing down whether or not he has to touch the wound is harder, but considering he literally reached out to touch a guy who could easily tear his arm off if he wasn't standing still like a moron, that's got some basis behind it too. Not much admittedly, but there's something that can be argued there. I could easily see it either way depending how stupid Kubo want's to be about it, and considering how utterly moronic Deicide was, that'd be VERY.
However in this specific case in question , it's very hard to argue against the fact that it's very deliberately a mass of hair clearly going up into a halo of light and firing it down to heal a guy. You can't argue 'dramatic effect' or the like with that, because it is SO utterly arbitrary that it can't be anything BUT important. You literally cannot assume beyond this without actual evidence to support that logic, otherwise that's just plain silly.
Which isn't to say there can't be some arbitrary facet to it that expands it's utility to cases where the hair is not available, but per canon we can't automatically assume it does because there is NO logical basis behind such an assumption. After all it outright requires you to go into full on pulling theories out of your ass territory to explain how such an utterly arbitrary ability works and why it should be able to work in any given situation outside the exact same scenario where it did actually work.
Keep in mind however the above is just in terms of arguing Canon. Fic wise you can literally do whatever the hell you want (Though without backing your shit up and clarifying your assumptions you best prepared to be argued on whether or not it's stupid), but in terms of hard canon the general line of though it is always going to be "If they haven't shown they can do it, they can't" in cases where it makes logical sense.
