Starfy is definitely the official localized name.
It sure is. Much like it was "Princess Toadstool" once. But localizations, "official" or not, are just that. Something beyond the original creators' reach, normally. I always thought that one of the good things of fan translations was exactly that -- faithfulness even if it opposes officialness.
While I know it seems the cool thing to bash on NoA and try to say that literally everything they do is wrong (which is the feeling I'm getting from such comments), it's hard to take A URL of a Nintendo of Japan website from (I assume) several years before any localization as "proof" of anything.
My point is: I doubt there was serious thought put into what they may want to someday call the character in English before they created that page, the spelling in the URL could have just as likely been made up by the webpage designer. We don't know, and shouldn't assume more than that.
You're right in many things there but you're actually missing the point. A Japanese author indeed will rarely put serious thought into the romaji form of a made-up name, even if it's actually an English-language name such as this. This is precisely the point -- they invent English-language names which are just written in katakana, so it's the labour of a good translator figuring out which is the best way to transliterate it if there's a lack of an official source.
For this very case, it's quite clear for anybody used to Japanese-English transliterations that they wanted an English-language name for the character -- they ended the name with a long -i ("sutafii") instead the short form ("suta-fi"), which would have been the natural portmanteau (and would have been a Japanese-language term more than anything, no matter where it comes from). An ending long -i denotes they were thinking about an English-language name ending in -y, such as Daisy, Mickey, Yossy...
So once we're sure they had an English-language name in mind (therefore, we can discard "Sutafii" or whatever), we have to wonder if the URL naming here is really an arbitrary thing (as they usually are in Japan, I'll grant you that too) or if, on the contrary, it's giving us the only reasonable way to transliterate the stupid name for once. The truth is, as I mentioned, that it can never be "Starfy" since the "a" is short.
I would say that at this point, even if NoA is given some freedom to localize stuff, they do have supervision from Japan and I would think at least MAJOR CHARACTER NAMES is something (these days) they value enough to be sure THAT is correct.
And you're going to have a hard time convincing me that "Yoshi" is solely "NoA arsdoing", considering even Japan uses "Yoshi".
So "assumptions" in this matter are OK? There's not "supervision from Japan" in this regard, be sure. A Japanese company just wants their games to sell in the Americas. If American marketing dictates that "Yossy" or "Stafy" are too alien for the pure minds of American children, they'll bastardize the names no matter what. Of course, Japan will re-adjust if necessary --and for a corporation like Nintendo it sure is-- but again, these days I thought it all was about "faithfulness". Fans grow up and evolve, don't they?