I want to clarify a bit more on this. Going point by point, giving my perspective on each of your points.
Over 9000... (Joke), but seriously he is one of the 3 Unsundered, which means he's older than History. And even though the exact time scale has never been stated, each era is just over a 1000 years, ARR starts the 14th era of history. Meaning the Source is at least 14000 years old and could be millions of years old, meaning Emet-Selch is at least that old, and since the Ancients were timeless beings, he's even older than that. He's at this point in the story the Oldest Being known, with the exception of dragons and alien robots which could be even older.1. He's old. That's made abundantly clear.
Familiar relationship is clear, but the read could be lovers, husband & wife/husband, best friends to the point of being Brothers/Sister of another mother. Jumping right in to one of these relationships because it reflects on your past is fine, but it is an assumption to read more than face value here. Personally because of the very clear queer coding of Emet-Selch and his obsessive nature, I read this as Husband and Husband, with them breaking up over the augment over how they planned to deal with the troubles facing their world. Emet-Selch wanted to Create a God, and Azem wanted to Inspire Heroism, which Ironically inspired Elidibus even though he stayed on to Summon Zodiark.2. He shows a favoritism towards the WoL. ...
This is not really a point, Emet-Selch's current body plan is his Garlean Version, which are all Tall Hyuran, as base Garleans are Hyuran Heads on Elezen Bodies at Max Sliders. With named Garleans all being buff Roegadyn Sized Humans. (Drools at sexy Cid posters), Ironically my Femroe is Taller than Emet-Selch by a large bit.3. The average player picks a fairly short character, ...
Ex-Lovers who still love each other will wage immortal wars to prove they are correct. Although Final Fantasy uses a lot of Mythology in the look and feel, and naming conventions, the series has always based conflict on reality and realistic type events which is a lot easier for people to relate to. Million year hate between Siblings is rare in reality, while divorce of lovers is an every day experience, and totally relatable to most.4. In my experience, brothers have the fiercest personal conflicts....
end of Part 1 of 2