Let's go over this:
1. Accelerator cannot react to lightspeed attacks. He reflects them passively with subconscious calculations, but he's not aware of this happening, nor it would do much good be aware of it or even see light in slow motion if you cannot move or act at those speeds. C'mon guys, this is not up to debate, Accelerator has never reacted to light, lasers or anything like that in the series.
2. Ryuko it's not being hurt with the same amount of energy she dishes out, she's being hit by twice the energy of her attack (Original force gets reflected at her and combines with the one her body already is affected with by Newton's third law of motion). This can be increased if Accelerator feels like it, his reflection can reflect strength much stronger than what it originally was (accidentally hurting a kid playing with him, people greatly breaking their arms trying to punch him or hit him with a bat).
2. Ryouko cannot hurt him with her attacks. On the other hand, Accelerator cannot see her moving or react to her given the speed difference that is clearly not equalized to prevent the usual "The other character cannot do anything, Accelerator stomps. Thread closed". However, as explained in 2, Ryuko can be hurt by her own attacks being reflected back at her with increased strength, and Accelerator can add to that reflection by modifying his reflection settings.
3. Ryuko's Regenerationn means she's going to survive this reflection. The damage inflicted on her likely means she stops for a second or so to regenerate the broken arm or whatever. However, even if the damage were minimal, having your attack reflected like that by an unknown opponent is very likely to give anyone a slight pause, if only to wonder what the hell has just happened. That pause is all Accelerator needs, and he can just raise his hands and use his long-ranged vector manipulation to freeze Ryuko in place, using the strength of the planet's rotation beneath him and maybe even by turning whatever strength Ryuko tries to use to break free against her. Speed advantage should be gone.
4. Accelerator has locked Ryuko in place, can he finish her off? I'm not sure. There's an instance where, to get rid of some regenerating Dark Matter Sister clones made by Kakine, Mugino shoots a Meltdowner beam at Accelerator, who grabs it, adjust it and then shoots it towards the clones
Accelerator accurately grasped the vectors of that attack. He adjusted and focused the vectors to give them new form and properties. And then he used it to attack all three of the attacking girls at once.
Sound disappeared.
There was only light.
This went beyond simply piercing through them or roasting them. They disappeared. Not even a single piece or 1 mm fragment remained. They were truly annihilated. He made sure they would never be controlled so unreasonably again. Those girls should not have been there, so he returned them to where they belonged.
Accelerator doesn't have Meltdowner by his side, but Ryuko is not Kakine. More importantly, from this we can gather Accelerator is not an idiot who's going to try the same thing over and over to not avail, and in fact, has experience with regenerating enemies. When simply ripping apart Ryuko a few times like he did with Kakine won't work, he will rip her apart in incrementally smaller pieces until she stops regenerating. Will this be enough? I leave it to others to debate.
As for the possibility of Ryuko figuring out Amata's trick...No, just no. First she needs to realize Accelerator is passively reflecting all of her attacks no matter how strong/fast or whatever direction they come from (you know, the alternatives any reasonable fighter is going to try first instead of simply assuming things), that his ability manipulates vectors, that his reflection is limited to a small extremely thin field around his body, that the loophole exists and then figure out the timing for the loophole, a timing Accelerator can change at will. There's a reason Amata, a Kihara with bs martial arts abilities and the person who knows more about Accelerator's power, personality and Personal Reality than anyone else in the world, was the only one to successfully do that, even as Accelerator tried to shift the timing multiple times.
P.D: As for the she can hit stronger than his reflection, his power casually produced his High 6-A feat in one of his more weaker forms. And while not accepted since there's no proof in the story proper, the author clearly doesn't made it an ability that can be "broken" by throwing more energy at it, considering he literally compares it to the ability to nullify all lethal attacks no matter how strong as two abilities with an infinite ceiling.