[Galin]

But within this conception Olorin isn't arriving as an Istar, complete with and Istar's function and duties, here.

'That Olórin, as was possible for one of the Maiar, had already visited Middle-earth and had become acquainted not only with the Sindarin Elves and others deeper in Middle-earth, but also with Men, is likely, but nothing is [> has yet been] said of this.'

So why not visit Nerwen and so on, as Olorin, and for his own [and Yavanna's] reasons. The collective Istari mission to combat Sauron is in the future from this perspective.

Also, I'm not sure Olorin the Maia had certainly known Galadriel in Aman, as a friend.

It's possible, and certainly possible that he knew of her obviously, but no former friendship is stated that I'm aware of, and the suggestion that Olorin walked among Elves unseen, or as one of them [blending into the background? or becoming friendly with the Elves, or some Elves?], doesn't exactly necessarily speak to making friends with every, even notable Elf.

Not that you said otherwise.

I can see the thinking here, I mean it's Galadriel, but it's one of those things which is unsaid as far as I know.