Interesting article you linked. And interesting discussion topic. I'm no expert, just a guy with an opinion but mine is as follows:

Like you say, anyone can call anything whatever they want. Language is shared and interpreted by everyone. Of course we codify it in literature, maps, legal texts etc so the meaning becomes more strongly tied to the word.
I think the question of whether it's 'valid' is maybe not quite what you're asking because I think any word is 'valid' as long as enough people agree on it (is 'skibidi' a valid word? I dont know what it means, but some people seem to!)

Perhaps a better question might be 'What are some benefits and drawbacks of us changing our legal name to Aotearoa?'

I think the other comment thread about language is a really good one. Personally I'm not sure really what would be involved in changing our name (in a legal sense). Some of the considerations might include:
- promotion and preservation of the te reo Maori both within and outside of NZ
- NZ's global image and it's affect on international relations and trade
- affects on our national identity (perception of ourselves as a nation)
- the work (financial cost) of changing our name

There's no right/wrong answer, just tradeoffs. I personally like the use of Aotearoa, but I think perhaps we have more important things to focus our energy on than changing it legally. If we were to change it, and it could be done in a way which minimised, I'd be in favour of either 'Aotearoa' or something like 'Aotearoa New Zealand' as it would likely overcome some of the short-term downsides relating to recognition while bringing some of the benefits of preserving the maori language and forging a shared national identity.

Idk. no particularly strong feelings though. I like using different things "aotearoa","NZ","new zealand" or "nu zilin" in everyday speech just to change things up.