Fate itself is Tirnoch's canWe 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.
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.
The Weave of Fate only disappeared cause it had to reform itselfAt 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.
Ventrinimo wasn't going to betray everyoneThe 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.