You Asked - and this is a right mouthful of questions!!!
some are going to be answered in the text, some in outlying material. I've a Light and Shadows short story forthcoming in Jana Paniccia and Julie Czerneda's anthology, Under Cover of Darkness, that will be out next spring, and I just signed a contract for another related Light and Shadows short story to appear in a British Anthology, probably slated for next year.
To Answer what won't spoil other material -
Dakar is of no relation to the s'Dieneval clan lineage. His is a wild talent, not tied by bloodline.
The uprising began in 5018.
Dakar's prophecy of West Gate in 5061. He was four.
The last Paravian departed in 5100.
Sethvir received the gift of the earth-sense - but it was a MAJOR shift for him. He would have spent a LONG time at Althain Tower preparing for that responsibility.
The first slaughter of the old blood swept the western kingdoms first. Those to the east - quickly followed.
Meiglin s'Dieneval was born just AFTER the fall of Tirans. She was fourteen at the opening of the story - and eighteen when the story ended. Dari was born in her nineteenth year.
These events occurred - AFTER 5018, but far before 5100.
Dakar and Dari - never met. She would have been sent thru Westgate by then.
Sethvir would not yet have his earthsense, but would have proclivities - been in training for the post while at Althain Tower, during Dari's raising. He was also, as you guessed, on and off, in the field. His central post at the Tower would have become fixed after 5100.
The Sethvir prior to the earth-lined awareness was in no way accountable to the person he "was" prior to arrival on Athera in the Second Age.
The Seven once went by other names. the changes in their nature happened drastically - upon their arrival at Athera, or at least, was begun to be set in train the moment they "connected" to the dream of the dragons. this is Another Story and far more complex than you imagine. Paravian presence and the dragons were catalysts for what already existed in potentia - but - the Sorcerers are not what they once were - important to realize this, that their past isn't relevant to where they are NOW.
Dharkaron Avenger/Daelion Fatemaster/Sithaer - all these ideas originate with Mankind's beliefs. You won't find them in Paravian mythscape - though the concepts are given Paravian terminology, their origin is not of Athera.
The law of the Major Balance, Ath Creator - these are terms for a concept that is not human centric. These concepts were not brought in by the Fellowship, or by Man, but would reflect to some degree human effort to ascribe meaning to what cannot be defined in a linear fashion.
You will certainly want to see Under Cover of Darkness's entry for more direct access to the material.