Hello,
Q1: what I remember is that they came to understand everything of why their world is that way and also understand why the king that moved to the island decided that better just erase history and begin from zero. By the time, they will became more influenced by the king memories and will begin to feel and think like him. Just remember Eren at the end of chapter 90, not a really happy image of him after seeing for the first time the ocean (i still remember my first time when I saw the ocean and I can tell his reaction is not normal for a human being trapped his entire life without knowing how it looks), he was sad and kind of resembled Hystoria's Uncle and her sister. I read somewhere that the author said this manga will not have a happy ending, so you can guess...
The manga still don't tell us the entire picture of what is the origin of this conflict, but we can take our real world just as reference. In every war there is no such a thing like the good side and the bad side, just the winner and the loser. In theory, the winner get the right to rewrite history because war is just the next step after the clash of ideas and desires can't be solved by debate and rationality, then force. I hadn't read about a War that only one side commit brutalities.
For example, what we always are told about the WWII is: Germans bad, bad, bad. Indeed, they commited atrocities. But why germans turned radicals? Why nazism got such a massive support? And you need to get back to the outcome of WWI, where the winners forced the losers to pay for the reconstruction of the winner side, defeated soldiers were turned slaves for the reconstruction and not allowed to return to their families, the defeated side were forced to give for free their resources and even blocked their access to international waters to do business. The population of the defeated side was starving for a decade until the rage was unbearable and resulted in the surge of nationalism, extremism. I don't blame them, if I were in their skin at that time I also would have voted for the party that promised to end that sh*t.
Most people will said: They deserved it, they lost the WWI and dared to start the WWII because of the outcome of WWI. And what was the reason for the WWI? And you can roll and roll back in history and is endless. The more you read about it, the more you feel sad and confused about what is good and bad, because each side have their reasons to fight.
And if we think about the outcome of Germany after WWII, isn't it something similar to this manga of walls, wars and hate? They have names that sound like german, some songs are in german, even Mikasa is from a "rare and extinct" race from Asia (reference to japanese after being bombed with nuclear warheads?) Can you blame the King wanting to stop it, forget it, wanting peace, even if that peace is imaginary and protected by an army of collosus Titans?
Q2: Don't know because I haven't read that spin off manga and don't have will to do it. But the one they saw sleeping in the original manga was the weird Pet of the Beast titan and turned to be a spy that told Zeke and company about the Squad comming to attack. That is how Zeke, Berth and Reiner were a step ahead when they arrived and that is why so many people died. That titan "dog" is not normal. Also, remember that the group of Hystoria, Ymir, Reiner, Berth and others were attacked at night in that castle by titans following the Beast titan command. So that rule about titans sleeping at night is not universal.
PS: OK, I talk too much LOL
Q1: Years during the Titan War (Eldians vs Marleys).
> 145th King concluded that there is no hope for humans, there is no end to the war, humans (Marleys) are cruel, and I think they were about to lose the war at that time. THUS, giving up and secluded most of the Eldians in Paradis Island. He is a coward king, as they like to call him.
>This ideology became a curse to every successor to the Progenitor Titan. Thus, making every successor as coward as the 145th King.
>This ideology "only" applies to Royal Bloods (Fritz lineage, i.e. Reiss'). Eren, no Royal, does not develop this pessimist idea. BUT, in the ocean scene in Ch90, seems like he's going downhill, hope not though.
That's all I can answer, not sure about the spin-off since I did not read it.

So, i started reading it a long time ago, and i forgot some things. Right now i don't want to reread it, but there are some things that just keep bugging me. Please answer me, Oh kind readers of this manga
Q1.
So there are 9 shifters. The Projenitor Titan (which was owened only by the royal family, and now-by Eren) can control all of the titans.
Eren's father and the other elodians that formed the rebellion against the marley's, strongly believed (and even found evidence in books) that Ymir was a good leader, that didn't annihilate the marleys, even though she could easily do that (during the hundreds of ears of suppresion).
Then why all the bearers of the Projenitor Titan didn't do anything?
Hystoria's uncle and her big sister (with black hair, whom she adored) could have easily do the job, and free their people, but they didn't do anything. And when they were questioned about it, they looked kind of guilty? They felt like they didn't deserve to fight back the marleys?
Q2. For those who read Shingeki No Kyojin - Before The Fall (takes place 30 or 50 years before the original story)
In the original story Shingeki No Kyojin - we know that the titans sleep during the night, they are not active. We all saw how they passed next to a sleeping titan during the night....BUT in Shingeki No Kyojin - Before The Fall, chapter 22-23, there are 3 titans who are very much active. Why is that????