Different than totenkindly , I don't think that Halbrand being Sauron was obvious at all, and also given what I know about the Tolkien story, Sauron never ever had a human form - he was an elf at the original story when he participated on the creation of the rings, right? I also find non-sense to see Sauron fighting orcs. I think Halbrand being Sauron is at least inappropriate in a lot of senses but it seems they wanted so much a surprise element. I mean, for example, at the end of the story he is seem at Mordor, yet he was fighting orcs, what he is going to do on Mordor, he is gonna say "hey orcs, last week I was killing some of you and helping your enemy... Now worship and let me rule you, oh yaaa!"?? Seems quite of ridicule to me yet I think that something along that lines is going to happen at the beginning of the season 2.
Maybe you didn't read much of the Internet stuff (good for you!

) But yeah, he was a clear frontrunner for Sauron from the early episodes, to the fan base (this is article is from August):
https://www.inverse.com/entertainment/the-rings-of-power-season-1-who-is-playing-sauron
Anar was never fair and seemed too obviously evil, and The Stranger seemed very clearly an Istari by presentation (like, new to human flesh, looked like Istari, had powers like Istari, and so on, along with the affinity for fire and halflings). Yes, the term is Istari, essentially angels/Maiar in the form of old human men. Maybe it is worth noting here that the Istari (incarnate Maiar) were specifically sent out by the Valar in a form NOT LIKE THAT OF ANNATAR previously, who had ended up being Sauron; they were true emissaries of the Valar, but actually were forced to appear as old human men instead, maybe to keep them humble for their role and also to contrast.
So that left Halbrand, and he was playing exactly how I thought he'd play it if he was going to be Sauron. I think it also was almost done as a counterpart to Aragorn, who looked very similar but ended up being a "good" person. Well, here's the same kind of figure who ends up being baaaad.
It's not something I'd argue much about, but personally I don't think this version of Sauron fits with Tolkien's very well. Sauron/Mairon was drawn to Melkor because of his efficient organization, and Sauron was known to hate disorder, and I would think that would cary over to his appearance. I can totally see him as appearing as an elf or man of high bearing, looking polished and beautiful, immaculate; I have trouble seeing him ever wanting to appear in this guise as Halbrand, it's not his style really at least in Tokien's description.
Sauron served Melkor in the First Age until Huan the hound defeated him (in the Beren / Luthien story, when Sauron turned into a werewolf to fight him). He thus was in hiding when Melkor was thrown into the void by the Vaiar.
He came back in the Second Age as Annatar, Lord of Gifts. I am on vacation so I don't have my books with me, but he supposedly looked like a Maiar or something and probably elvish, although there's a possibility he was also a very radiant Numenorian. Whatever he looked like, it was enough to sway both elves and men who saw him, to believe him to be an emissary of the Valar. He wouldn't have looked like grungelord Halbrand, who again I feel like was modeled after the appearance of Aragorn to trick the viewers (because Aragorn looked a bit foul but was pure of heart).
1 - None of the 5 original Istari had ever arrived by meteor, right?
I don't think we ever get a description of them arriving, they just became known to people as they wandered about, interacting with others. Basically you would have heard of them as rumors, then eventually maybe run into one of them. There was no specific arrival -- the insinuation is that they might have landed by boat at the Grey Havens, from Valinor, and then wandered through Middle Earth?
2 - Sauron never had a human form and was an elf or elf alike when he forget the rings, right? Also, were not the rings forged outside middle earth?
I don't know what his Annatar form looked like, race-wise, as per earlier, but they thought he as a representative of the Valar -- someone good.
I thought all the rings were made together in Eregion. Annatar and Celebrimbor and other smiths made the Nine and the Seven together, but Celebrimbor made the Three in secret because he eventually didn't trust Annatar. Sauron secretly made the One in Mt. Doom in Mordor, and I guess despite his never having touched the Three, they still fell under its power because they were made using the same methodology/process used for all the other rings so they were affected by it.
Sensing Sauron's duplicity, the elves immediately hid the Three and tried to stop using all the other ones, which pissed Sauron off; he recovered the Nine and the Seven when he went a-conquering (so he could recover all the rings made to serve him), which he then doled out to Men and Dwarves to corrupt them, and of course they fell under the sway of the One. the rings weren't doing him any good when the elves refused to use them, so that's why he took them. But of course he never touched the Three.
There were also minor rings of power made, leading up to the twenty major rings. It would have been an interesting show to follow the exploits of people who might have found these lesser rings if they still exist. Now there they could have just made up whole story lines, without fear of running afoul of established lore.
3 - Is that battle the real origin of Mordor? I thought Mordor was there much earlier originally, right? I also agree that origin seems weird - first, it is weird that water causes explosions to the Volcano rather than just evaporate and even stranger that the volcano got active almost forever after that.
Yeah, it was weird. It should have just cooled down the magma. At most the steam could have blown the top off the mountain -- a one -time event.
Mount Doom existed in the First Age, supposedly made by Melkor? Melkor was going through the world, adding fire to it as part of his machinations (from Tolkien's notes published later). The elves called Mt. Doom "Orodruin," which is basically burning mountain.
Not sure when all the landscape was changed to look like a box, but that was a Sauron and/or Melkor thing? I don't think that the land looked beautiful but always a box, and then suddenly it was turned all fiery and stuff like in the show.
4 - Sauron and that other elf created all the rings (except the main one that was just Sauron), right?? Not just the 3 rings of the elves.
Yes, see earlier. Note it should be expanded to include other nameless "elf smiths" along with their leader, Celebrimbor (who was Feanor's grandson, by the way -- the guy who made the Silmarils). They forged a lot of practice runs (minor nameless rings), and then the 16 rings (that were eventually given to Men and Dwarves), and then Celebrimbor forged the Three alone in secret (so Sauron never touched them), and then Sauron forged the One alone and in secret.
5 - Is Isuldur on the right time line, I mean actually alive at that times? Or they just added him to have one more familiar name on the story?
Well, Elendil and son Isildur (and the younger son Anarion -- I can't tell whether they changed him into the sister Erien, or whether he will show up later) were born a lot later. The timeline is all wrong. Here are some dates for you (SA = Second Age)
- 1500 SA = Forging of Rings begins
- 1600 SA = Sauron makes the One ring
- 3119 SA = Elendil is born
- 3209 SA = Isildur is born
- 3319 SA = Numenor vanishes beneath the seas, only a few ships (w/ Elendil's clan) escape
- 3441 SA = Sauron is beaten and the One ring cut from him.
Note (1) Elendil and Isildur do live pretty darn long, as they are closer in the tree to Elros, the seed of their line, but (2) the rings have been around a LOOOONNG time before they enter the story.
6 - Did orcs really were like vampires on the original story?
My understanding is that the orcs were created in a time when the sun did not exist and they lived in darkness, were born under darkness. When the sun finally appeared, they hated it -- it blinded them and the heat felt like it burned. However, this is more a psychological thing. I am not aware of them actually being hurt by the sun, it was just completely counter to their creation environment and the environment they were acclimatized to. I think in LotR they didn't really much like the sun and tried to stay out of it when possible, although I think the Uruk-Hai weren't ever really bothered by it (they were bred in Isengard in a world where the sun already existed) which is why they were a big deal.