Ok, some thoughts after reading through the whole thing.
1 Someone has already mentioned the possibility of the dark titan showing up in the final battle. Can he be tricked into fighting the good guys, or will he always prioritize whoever has the ring?
2 If I were running the campaign as a player, I'd surely start a side project of trying to turn the Nameless King's machine into a weapon of mass destruction (linking it remotely to the royal scepter, so it could dump the accumulated negative energy onto the battlefield.) Do you think it would work?
3 In the "two solars" version of the final battle, the PC's allies flee, except for Grumblejack. What about Sakkarot? If the players take him (obviously disguised) to fight by their side, would he flee too? He seemed like a very fatalistic type.
4 It bothers me a bit how the whole might score thing ends up largely cosmetic in the end - no matter which side is outnumbered ten to one, it will be down to the fight between the leaders in the end.
5 There are two courses of action that seem very logical to the players, but aren't described in the book at all.
5a - countering the princess' efforts in the mainland. Yes, she's permanently veiled, but a gathering army of thousands is much harder to hide, and if Dessiter gets some intelligence info like he was supposed to, the players might come up with a plan to delay her - hitting (or bribing) her potential allies, or perhaps persuading the kraken to harass her fleet en route.
5b - search for Anthea's lair. Even if the players haven't figured out she's their enemy, her treasure is the last major unclaimed hoard in Tallingarde, and the players are conquering the north either way. With some initial clues from Polidorus' book on dragons, they shouldn't have much trouble pinpointing the lair - and I'm pretty sure it wouldn't just be a frozen cave with a pile of treasure and a bunch of guardians. I really wish there were some more descriptions for it.
So, has anyone tried writing up Anthea's lair?
I'll give you some feedback:
1) The Thanatonic Titan from the Ring is basically an evil genie, in the sense that, when freed, i can be a powerful ally like a terrifying enemy. He's extremely intelligent and possibly somewhat aware of the situation of Talingarde throught the ring-wearer senses, if you so decide. However, by design, he's an enemy boss. If freed he should try to claim dominance over the land of Talingarde. If this requires to ally with the Pcs for a few fights, he will do so, but be sure he'll greater vital strike any creature that might oppose his rule the second the princess's forces are defeated.
2) The machine is not a cannon, unless you actually build a cannon over the Aghatium. Which is complicated. However, by design, the machine could be use to "create" and "bottle up" negative energy elementals, which could be deployed as weapons in a battle. Other, more sofisticate uses, are up to you.
3) The "two solars" scenario is build to stagger your players giving them a CR 25 fight, but since no creature exist with a CR label clearly visible, very few of the Pc's allies could infer their power, and therefore fall into despair. Who escapes this battle, if any, it's up to you.
4) By my understanding, the adventure it's heavily focused on getting control over Talingarde, which gives pcs little time to look outside the island for the princess. Nevertheless, you could arrange a number of situation, like discovering a few named npcs that your pcs may discover have been visited by the princess, and that may actually be interacted with.
As as example, i would suggest: 1) a monarchy allied with house darius 2) some other dragon (i remember the name of an ancient gold dragon offered by mr Mcbride), and possibly some kind of Azata/fairy princess.
5) Antharia's lair is a good place to visit, but should be almost empty. Antharia is, by the book, currently plane hopping with her daughter searching for allies. However, if you want to give your Pcs something EVIL to do, they could discover inside the almost void lair some kind of massive forge were Antharia's colossal mithral armor is being forged. They could either destroy the forge and steal the mithral or, if they are smart, try to intercept the dragon when she comes to put it, or even put high level curses on the piece, so that when the dragon wears it something very very bad happens - possibly during the battle, so that Antharia does not have time to counter it.
So, has anyone tried writing up Anthea's lair?
Not that any of my groups ever got even remotely close to this section, but I always had it ready to literally steal the silver dragon lair design from the 3.5 Draconomicon (page 266) and went with CR 18 dragon horde wealth (page 283), not including the mithral armor being created.