Okay I'll try and explain this for you. Firstly, understand that nobody is trying to argue or convince you that black people aren't being inconsistent or hypocritical with the word. We are trying to explain to you that they are being exactly that and that's part of the reason white people can't say it.
When I said "the two variations of spelling are not deemed as different words by anyone." I was referring to how the word was viewed before the mod nineties. Nigga was just a different way of spelling nigger on hip hop albums originally. When blacks first started calling each other nigga they still spelt it nigger. They continued spelling it nigger, regardless of how it was used, for decades.
The spelling thing came mostly with hip hop but it didn't make the word any different. That would be like saying 'chains', 'chainz', and 'chain$' all have different meanings. Poor grammar does not equate to inventing new words.
The notion that the two different variations of spelling are used for different uses came later and mostly as a way to justify its overuse in the media. So the societal perception for some is that the spelling defines the use. It is actually the other way around. How it is used will determine what kind of spelling it may have. But the truth is you can use nigger any way you want and spell it either way and everyone will know what you mean. The spelling is irrelevant. And again, intentional grammatical errors aren't new words and saying it's a word with a different meaning when it applies to one demographic but still the same when applied to another doesn't actually make it a word with a new meaning.
Just remember, people aren't trying to convince you that this is a rational decision. They are trying to explain to you why whites shouldn't say it. Part of that reason is based in irrational hypocrisy. Like a lot of things in the world. You can't make complete sense of it because there is none. But regardless of spelling or intent, nigga/nigger will still be perceived as being the same racist word if a white person says it. Just like if a black person says nigga/nigger it's still perceived as the same term of familiarity. No it doesn't make complete sense. Yes this is why whites shouldn't use that (those - if you prefer) word(s).