the assumption that all he had to do was react when the spell was first cast is irrelevant to the explanation of the scene. But before explaining this point I'm going to summarize the scene as a whole, because after rereading the scans there are presuppositions of t'a par that haven't been demonstrated or that are no longer relevant, taking all the panels.
1 - The first action is Kezess activating the "static void" spell -> it's a time stop spell based on omnidirectional propagation (via Arthur's descritipn who describes the world progressively freezing with the propagation of the wave)
-> Quote : "Kezess made a short, sharp cutting motion with one hand, and aether flared outward from him, rippling through the atmosphere, causing the world itself to harden and go still. The mana particles drifting in the air were motionless, and a string of herbs, which had
been slowly rotating in the subtle air currents, froze."
2 - The second action or important element is the wave reaching Arthur's coordinates, so the wave is at his level, Arthur describes that he feels it. Then he describes the time beginning to freeze -> this is logical, as the wave is at his level and the wave is the sign of the propagation of the time stop. In his description, he semantically adds the element of the time stop at the same time as the wave's propagation, and this will be explained with the different steps.
-> Quote : "Then the ripple rolled over me, and I felt time stopping"
3 - The third element that will answer your “debunk” on why Arthur didn't react at the beginning. While Arthur is being hit by the Time Stop wave, he has a flashback in which he remembers Elder Rinnia managing to escape it, and it's after this flashback that he pulls himself out of the timestop. But basically, he didn't react before because he didn't have the knowledge that he could to resist the attack because it was the mental unblocking by thinking of Rinnia that enabled him to do so, and this will be demonstrated in the 4th action.
-> Quote : "My mind flashed back to a time before the Relictombs, before my draconic form, before Sylvie's sacrifice.
I remembered sitting with Elder Rinia. I'd been suspicious about the nature of her powers, and so activated Static Void without warning. She'd used aether to counter me, freeing herself from the time-stop spell."
4 - Arthur describes how, using his instincts, he wrapped himself in Aether, enabling him to extricate himself from the wave that was coming into contact with him. My explanation here is simple: given that he escaped from the wave that was described as surrounding him, this means that the wave didn't completely pass through his position, otherwise he wouldn't have been able to get away from it. In action number 2, he was in the process of catching the wave, but at no point was it specified that the wave had passed completely over him. In short, there's a difference between having the wave at your level and having it completely overwhelm you. Knowing that, as explained in the description of action 2, Arthur began to feel time freeze on his person precisely because the wave was at his level and unfurling on his body, but this again doesn't mean that it completely overtook his body if it was unfurling on him. As it spread over Arthur's body, he reacted by pure instinct, just before he was completely buried, and that's why it's explained when he says he escaped the wave. He couldn't have escaped if he'd completely passed over it because he escaped by putting Aether on his body -> his body didn't move.
-> Quote : "Reacting on pure instinct, i pushed outward against the ripple with a burst of my own Aether"
Then I'll move on to your last point about describing events, and I'll use the example of classroom of the elite, which is what you used as an example to try to debunk Arthur's perception through description. In Classroom of the Elite in Volume 3 of Year 2, in the island arc with Kushida's POV when she confronts Ichika in the forest, she gets blitzed and we as the reader don't get a description of Ichika's action, it's after she's been hit that Kushida assumes via several factors (pain and marks) that she's been hit but this is presented to us after the action has taken place. In Arthur's case, there's nothing to make us assume that he didn't perceive the time stop, either narratively or semantically.
To sum up, Arthur has indeed reacted to the time stop and this shouldn't change anything in the calculation, except for an increase.
