"Our famous director delved deep into the ideas embodied in a mysterious wizard trope and reinvented them in a way easier to immerse in for modern audiences." Or something like that; don't let a couple nerd fans dictate some minutia of how to make films that earn actual monies, which we know much better! And we don't need read no books when there's extensive visual material already available!

So you're saying it's well within realm of the lore for the immortal angel sent by the all-powerful diety to take the form of a young, fit hollywood celebrity and kick the face of orcs as part of a three part imax trilogy? Actually, and I don't want to annoy too many people, but with this context, I think they should go all out and make Gandalf team up with other angels in the form of butt kicking women, a fantasy charlies angels, where Charlie is the diety. I doubt angels in that lore have gender anyway.

Fantastic Wizards, and where to find them.

Not visible - he probably took part in the last battle against Morgoth, but that was as one of the many, many Maia that also took part.

Istari are sent after the events of Silmarillion end, after War of the Ring.