Acnologia took the full force of Igneel's claw anyways. He was hit square in the face by Igneel's claw. The fact that he was hit probably meant he was caught off guard. Igneel caught Acnologia off guard with his roar as well. There's no evidence that Acnologia flinched away. The roar also engulfed him, which meant it must have hit every part of his body - including the eyes and whatnot. Even if rolling away can mitigate some damage, it shouldn't be that huge of a factor. If a tiger claws you square in the face, there's only so much damage you can mitigate by rolling with it. The fact that Acnologia didn't receive any injury from the claw meant that this attack never penetrated his scales.
Igneel's claws are sharp. Why would it not be comparable to a bladed weapon? Wendy's claw attack can already tear human flesh, arguably better than some sword attacks in this series. Acnologia himself can gut God Serena with his own claw attacks. Igneel's claws in dragon form should be way stronger. The damage Erza deals against Acnologia is proportional to her attack power versus his durability. Erza had a free cut against Irene, whom made no attempt to dodge whatsoever. It still did less damage than Igneel's bite against Acnologia's arm.
Just because the overall size of their body is larger doesn't mean losing an arm isn't a significant injury. I don't buy that it would be any less painful. We have larger bodies in proportion to our limbs compared to certain animals. Doesn't mean chopping off a limb is any less painful. Losing an arm would hurt regardless whether Acnologia was a human or dragon. It takes a lot more to damage a dragon than it does to damage a human. Once damaged, there's no reason a dragon wouldn't feel pain - especially from something as serious as a lost limb.
No matter how I look at it, it's quite obvious that Acnologia vs. Igneel is not the same situation we have here. You can't apply those feats to this battle because Acnologia was fighting and attempting to kill Igneel.
And clearly, the roar didn't directly hit Acnologia in the eye, otherwise he would be blind by now.
If Igneel's claw could make some cuts on his chest and torso, then his roar
certainly would have burned Acnologia's eyes out.
That's not the case
because Acnologia never let Igneel take a clear shot at his eyes.
Here, Acnologia is
immobilized, the injuries that Acnologia sustained against Igneel are hardly comparable to having both of his eyes slashed by a sword a point-blank range.
And again, in the rest of your examples, Acnologia
was not immobilized. He was fighting against his opponents, which would mean he is defending, dodging, or blocking their attacks. He is also bracing himself for whatever is sent his way.
The sharpness of the claw is really irrelevant to be honest. All it takes is the sharpness of a kitchen knife to make you go blind.
If Acnologia is immobilized, he can expect to take a sword to the eye as deep as his sockets would let it.
There is no way a dragon losing an arm is the equivalent of a human losing an arm...
There is a reason why Acnologia was still able to fly hundreds, if not, thousands of miles with a missing arm.
Most humans would be yelling in pain and kissing the floor if they had their arm cut off. Clearly, a dragon's physique is
a lot tougher than a human's.
They're more tolerable to pain in their dragon form than a human who received the same injury/damage. Hence why the pain felt by a dragon
cannot be compared to that of a human.