You're making assumptions as to what Tolkien meant by "amazon," when in another letter he clearly defines it as a female soldier. He was a scholar and fan of myths so what else would he use the term "amazon" for if not in reference to the warrior-women of Greek myths? In later texts Galadriel most certainly fought at the first kinslaying and there are several times she would have fought defensively (as all elf-women do at times of need,) as well as there being at least one version who would have answered the call of Eonwe and fought in the War of Wrath. As one of the most powerful elves (if not the most powerful elf) in Middle-earth, a leader who travelled frequently, considered herself a commander and was concerned with the dark stain left by Morgoth from the start of the Second Age, it is difficult to see why Galadriel would not have found herself involved in battles throughout the lands. Out of interest, in your mind, what "magic" did Galadriel use to tear down a fortress (I assume you mean Dol Guldur?)
Depending on which ageing system you use for elves Galadriel remained in the life-stage Tolkien referred to as "youth" either until the end of the Second Age or right through to the end of the Third. Using a young actress is not inappropriate and having her appear younger than Gil-galad is an issue with casting humans to play elves, the only casting decision I really have an issue with is Celebrimbor though I'll be interested to see if the try to justify him appearing older in the series or if they just liked the actor.
I don't know how Annatar is being introduced in RoP for you to say who suspected what and when, what is made clear in her story is Galadriel was most acutely aware of dark forces moving in the wake of Morgoth's defeat, though at that time no-one knew it was sourced from one agent who would be revealed as Sauron.
In general though Galadriel's character was very much in flux throughout Tolkien's life and her exact actions during the Second Age no doubt would have been made clear if he had got around to rewriting the Silmarillion as he intended. It's the uncertain nature of the Second Age that makes it fertile ground for telling stories without rewriting works that Tolkien had already set down as "canon."