Agreed, it just seems silly to have made it so vital to the game's story - but I think I will just watch it like you suggested, here's hoping you are right and that it does do a better job further on.
Maybe my opinion on this will change as I progress, but I really don't think it's that vital at all. As least not as far as information goes. Sure, you won't actually see the invasion, but we knew about it no longer being the game's opening before we knew about Kingsglaive.
Anyway, they already tell you this in Chapter 1:
- Lucis is a kingdom, Niflheim is an empire, they're at war
- You're the prince of Lucis
- Iedolas Aldercapt, emperor of Niflheim, is visiting Insomnia, Crown City of Lucis, to sign a ceasefire in the upcoming days
- The terms of this treaty are:
1) Lucis must surrender every area beyond the walls of Insomnia, namely Leide, Duscae and Cleigne (there's also Galahd that's mentioned in the movie, but the game doesn't even mention it, as it's irrelevant except for providing background for movie-only characters)
2) The Prince of Lucis and the Princess of Tenebrae are to marry, as a symbol of peace. This marriage will take place in Altissia
- The people are not too happy about this, but they don't seem to be that angry either, and it's understandable, just look around in Leide. Lucis has gone to shit outside of Insomnia, the war tore the place apart
- Nights are getting longer, and daemons roam the lands at night. This event is known as the Plague of the Stars, and will play a major role in the world of Eos. Wildlife is already being affected and mutated by it, even during the day
- Noctis and Luna know each other since they were children, it's been 12 years since they saw each other, but they still write. Even though it was a marriage by convenience, neither seems unhappy about it, just a bit nervous
- Luna is also the current Oracle, the one who communes with the Astrals, who are gods in this world, said to slumber to gather power in order to fight the Starscourge
- The Crystal of Lucis has a will of its own, and lends its power to the worthy heir to the throne through the Ring of the Lucii
And I didn't even finish Chapter 1.
Now, of course you can still say that the storytelling is shit, and you really don't like the way this information is delivered, but saying it's not there is just not true. It's one thing to criticize the way the information is conveyed, another thing entirely to say it's not even there.
Kingsglaive just shows you this, instead of you hearing about it. But we knew the Insomnia opening wouldn't be playable before we knew about Kingsglaive. You can see the party hearing about the peace treaty negotiations on the radio during their roadtrip in the TGS 2014 trailer. Last time we actually saw footage of the Insomnia invasion with Noctis was in 2011, when it was still Versus XIII. They talk about it in the E3 2013 trailer, but the action sequences are from Altissia and the tech demo square. We did get to see Noctis in Insomnia in the beginning of that trailer, but not the actual invasion.
I do, however, completely agree that it should all be part of the game, not as long as it is, just a few shorter cutscenes, even if Noctis isn't present.
But I still think that Brotherhood is more important than Kingsglaive. It even talks about the warring factions too, even if only briefly.