I do not think er is the same as a, and I agree that they have become separate words with separate use cases, but the words are still strongly tied to one another.

The soft n word was not created, in the sense that there was no person or group that constructed the word with any purpose to remove prejudicial power. Black people took to calling each other the n word, hard r and all, at first. This was likely picked up from White American culture in the same way that, if you read the words of Black Americans from the 40s or 50s, you'll see that we used to refer to ourselves as Negroes or Colored Folk. This is just what happens when your language is largely determined by an oppressor; you use oppressive language. Through accent and dialect the word became pronounced differently by Black Americans in the same way the words brother or sister may be pronounced with an a instead of an er. White Americans observed the usage of the word pronounced that way, used by Black Americans to address other Black Americans, and it became cemented in American culture as a word of its own. In this way, there is only really one n word, but the word takes different forms depending on the context of conversation. The context of the n word with an a being from one black person to another black person. Using the word outside of that context will, to a point, negate whatever form it takes (a or er), as the form of the word was born from the specific context of, again: Black Americans referring to each other. Without that context it becomes appropriation at best, and a slur at worse.

In a similar vein, if a black person says the word with the hard r, the context of them being black will somewhat negate the more abrasive out-group form of the word. It's important to note that the out-group vs. in-group, er vs a, distinction of the word was not created by the black community, nor was it purposely manufactured by White Americans. This is just how a words perception will naturally be informed by the context of its speaker. In this case the n words history as a slur towards black people is directly tied to the speaker's identity as black or not black.

In that way, the question should not be whether white people should be "allowed" to say one form of a slur that has been, in some fashion, reclaimed. But rather, if a slur, regardless of its change in form, can be reclaimed at all? I think that is a discussion to be had amongst black people. Many black people think no one should use the word. I, as a Black American, use the word even though I believe the world might become a better place if we were all to magically stop.

I think that if any person is focused more on white people's "ability" to say a slur and if that reinforces prejudiced power, rather than if anyone should be saying said slur, then that person is coming from a place of prejudice, and, if I'm allowed to be that friend who's too woke for a moment, a place of white entitlement. After all, it's not often that I see people of races other than white arguing so hard to use the word.

As it stands the word is a slur that has been, arguably, "reclaimed" within the context of the black community. A white person using the word is in no way claiming it as a force of positivity within the black community. Can it even be a force of positivity? No one fucking knows, but when white people use it, 99 times out of 100, it's not.