WMG / Kingdoms of Amalur: Reckoning

Fate itself is Tirnoch's can
We know that she had to create the Fateless One in order to escape, which implies she was fated to never escape. Maybe she could manipulate the tapestry to a certain extent—creating the Tuatha and so on—but couldn't flat-out turn it around; she was still bound by the rules. This, likewise, explains why Fate disappeared when she was killed. It wasn't that she was the creator of the weave, but rather she was the purpose, so without her the tapestry dissolved.

So basically, Tirnoch was fated never to be freed. But once she was freed, she was fated never to be killed. Which is why the Fateless one was the only one who could do anything in both cases.

Tirnoch is also Lyria, one of the "official" gods.
Some more zealous followers of Lyria (one town in particular is especially dedicated to this belief) believe that all events occur as per Lyria's will, and Lyria makes bad things happen to people as punishment for bad things they have done. It is therefore implied that Lyria weaves fate. Tirnoch is the only being, asides from The Fateless One, who can change fate, and without direct intervention, fate's course cannot be changed from what Tirnoch has woven. So, if Tirnoch is the only being that can change or control fate, and is also called a goddess, and Lyria is believed to be the only one who weaves the fate of all beings, then it's quite logical then that the two are one and the same, or if not, then they are the same type of being.
  • Mind you, any fateweaver who was not a follower of Lyria would tell you that what the followers of Lyria is spouting is complete garbage: the fate of all beings (even the gods) was decided well before anyone, anywhere, even the gods, were born, and cannot be changed no matter how many times you beg the gods for intervention (Tirnoch and the Fateless One being the only exceptions). Besides, if Lyria is the one who actually decided the fate of all beings, and her weave decrees that the Tuatha will wipe out all mortals everywhere forever with no way of changing that fate (and the fae will follow soon after), why would anyone worship her, since she is obviously the enemy of all mortals, nay, all living things?
    • Obviously, the followers of Lyria never made that connection (or refuse to acknowledge the possibility that their god is either just as bound by fate as they are, or an Omnicidal Maniac that is out to kill everyone, even her most devout followers), including the fateweavers who follow her, and should know, from experience, that fate cannot be changed.

If The Fateless One had not been born, the world would have inevitably been destroyed.
Consider the sheer number of disasters coming about in Amalur as of the game is opening. Nevermind the Tuatha, who are bad enough on their own, there are multiple cults trying to summon Niskaru Lords (nigh invulnerable beings that can ravage continents singlehanded), ancient fae wizards and witches coming back to life to wreak havoc on all mortals, the Dark Empyrean emerging from her sealed stasis, etc. etc., then even without the Tuatha, if the Fateless One had not come into being, then this era would have essentially become Götterdämmerung, as monsters, evil wizards, and demons from other dimensions basically annihilated everything everywhere.
  • What makes this especially dire is that there is literally no one else that could stop these events. In more than one case Fate is explicitly preventing anyone from solving the problem (the warsworn questline requires you to unbind a fate-sealed door that will allow no one but the fated one to enter and obtain the hammer. Since a Niskaru lord is preparing to enter the mortal realm, and the hammer is the ONLY thing that can stop it, period, and there is no chosen one anywhere who can open the door, Fate is decreeing that said Niskaru lord is going to enter the mortal realm and ravage the world, and no one can stop this from occurring, basically making the hammer completely useless unless fate is unbound). Even without Tirnoch's emergence, the world was essentially screwed with no help of salvation, until Tirnoch caused the Fateless One to be born. Nice Job Fixing It, Villain!.

The Weave of Fate only disappeared cause it had to reform itself
At the end of the game, Agarth says that he can't see the weave anymore. This may be due to the actions at the end caused such a massive change to it that it was impossible to read while it reformed. Fate still exists, only at the moment it's all in the air.
  • This also explains why the Fateless One can still use "Reckoning". The Weave's magic still exists, but it's just hanging around as an incredible source of power rather than actually dictating anyone's fate. This might also explain the rise of magical potential in the "Age of Heroes" — everyone is tapping into the Weave's power.

Ventrinimo wasn't going to betray everyone
The reason he was killed because he was figuring out the truth about Tirnoch. In the last conversation the Fateless One has with him has him theorising why the Player Character was resurrected- this was close to the truth, so he had to die. Ventrinimo was Killed to Uphold the Masquerade and the tale that he was going to betray everyone was lie that sounded reasonable at the time.