That's a pretty good synopsis.
I will impart a few "clarifications" however. Pardon me, if I accidentally reveal information that is in Book 3...the dividing line of 2/3 was a bit fuzzy and moved around.
Most everything here, I have said somewhere before (albeit most likely in the beta for book 2) So beware of accidental spoilers. Consider the following as sort of pre-beta info. It's not so much plot as clarifying underlying issues.
Technically, the Nyjyr Ennead are on Uropia, the closer of the two moons. They purposefully shutdown their outer planes to save energy. They have only been in the Abyss for the last 150 years (when Tizzy showed up with cookies). This is one of their multiple plans to regain power. (Reggie is another) She is posing as a demon to get her hands on "The book" and playing Exador and Ramses off each other. Of course, each of them is doing the same as well. Not a trustworthy lot.
Yes, there is quite a bit of crossover between the Gods of the Grove and the Nyjyr Ennead. However, that's not necessarily the same as the Ennaad and the denizens of the grove.
The Grove is a safe haven for people/races persecuted on other planes; think of it as sort of an underground railroad to other planes. Yes, when the other worlds the Grove rescued people were cut off (Earth) is somewhat coincident to Ragnarok, but not that simple.
Ragnarok essentially precipitated the eventual disconnection, by locking most of the Northern European deities out of Earth This gave the Demi-Urge a big opening.
The closure came because the Demi-Urge was in the process of slaughtering non-humans on Earth and hoovering up all the mana. They got out who they could, and eventually it was just too much work to keep doorways open. So for example, Merlin and Arthur were some of the last to escape.
It wasn't just the snakes leaving Ireland.
They have representatives of multiple races, not all of whom always get along: Case in point the Los Alfar and the Modgriensofarthgonosefren who are, technically jotunkind. So when the FOG decides that other jotunkind are evil, the Modgriensofarthgonosefren get uneasy, and one cannot assume their loyalty.
In particular, a battle between Orcs and Elves is dicey for them and their own internal stability. While not allies of the Orcs, they are not enemies per se either. So, if the alfar start arming for war against orcs, without true provocation, the dwarves will not be happy and likely escalate. This is was Trevin is concerned about; a war between elves and dwarves would be very very bad, considering both races crew their cloudships and defenses.
Yes, the grove predates Ragnarok.
Ragnarok was big on multiple planes, far beyond even the localverse, given that Earth was involved. In the appendices/library when it talks about war between pantheons, that's probably what the author was talking about.
Ragnarok lasted for very very long time. Around a thousand years or so, depending on which battles you include. It manifested itself in many ways on different planes. There were, to say the least, a buttload of battlefields in the war. As we will see, the "Desolation" of Ferundy and Cal Crestor is tied to it; and this was a huge war between orcs and elves.