First, let me explain why A thought your behavior was (perhaps bordering on) a personal attack. You're using a particular style of argument, where you get someone to imagine something that is very clearly awful in order to show them why something else is at least a little bit bad. Therefore you are both saying this person is wrong, and trying to upset them. Often this is phrased in a way that suggests the person who doesn't object to the mild thing also wouldn't object to the awful thing.
Consider: we were talking about people who walk too close to you on the sidewalk, and A said it didn't matter, and I said "what if someone bumped into you and knocked you over and your phone got smashed and your clothes a little ripped and people saw up your dress?" It's designed to be upsetting and to show the emotion that some people are dealing with in that situation. It's also designed to get A to change their mind when they say they don't mind something or it doesn't matter.
So, in your case, A was being all tolerant and accepting and "ok, some people don't quite get the nuances of names exactly right, but it's fine, I can handle it" and is probably feeling quite pleased with themselves. And here you give a deliberately upsetting extension, calling someone by the completely wrong name, and ask "how would you feel?" to further engage A with this example of name errors. You perhaps cause them to feel you're saying they say it's ok to get someone's name completely wrong. So their calm feeling of "being on the side of good" is interrupted with something that upsets them and tells them they are wrong. A felt attacked as a result.
The correct response in the very moment would have been something like "sorry, that wasn't my intent" and then continue the conversation without "attacking" A again. The moment for that has passed, so I would put it behind you and not mention it again. That includes not apologizing for it if A is not indicating any ongoing discomfort. If you think there is some discomfort a simple "sorry for getting carried away in our argument about names, I didn't mean to attack you at all but I know I spoke too strongly" will probably convey your position quite well.
However you should learn from it. You can make these remarks without upsetting people. You need to warn people that your exaggeration is coming, and give them an opportunity to say they don't support your exaggeration. Something like "mm, I can see you want to tolerate a little mispronunciation here and there, but there's a limit, right? For example if someone were to call you not A, that would be too far, right?" Here you "meet them where they are" then make your point. They can agree with you without letting go of their original position. You can then either continue the discussion and see if the two of you actually agree, or at least establish how close you are to agreeing. You're not "putting words in their mouth" that they agree with your extreme example, and you're not surprising them with an upsetting example either. As a result they are less likely to feel attacked.
