Tinalles, I really like your Vallik idea. The idea of a character who is legendary in a society for being one of their greatest heroes and one of their greatest villains is so much fun. I like it so much that I wouldn't want Vallik to be fooled offscreen by Naqualia so that the combat at the end of book 4 happens as written. There's enough inherent drama that I'd want my players to be able to interact with Vallik more extensively.
My impulse as a GM would be to put Vallik at a point of crisis: he's realizing that he can't truly be a protector of Talasantri if his secret is placing the city at risk. Maybe he's also coming to think that he can't truly be redeemed while hiding who he is, because as it stands everybody knows both worships him (as the Wavewalker) and despises him (as the Blackhand) and neither reaction is entirely deserved. The point is that he's suffering serious doubts about his life and doesn't know what to do next.
I would introduce him (under one of his guises) early in the book and have some kind of relationship develop between him and the PCs. (He could be 'helping' them look for Vallik, for example.) If a friendship naturally develops, then maybe he confesses everything. Alternatively, the party could discover that he's got a secret and confront him. In any case I'd have the end of part 3 be a role-playing encounter between the PCs and Vallik. If they can provide him with a way forward, then he sides with them against Naqualia; if they react badly or roll badly, he gives Naqualia whatever she wants (including the PCs) to keep her from harming more citizens of Talasantri.
(There is the question of how Naqualia finds the Alabaster Trident if Vallik sides with the PCs; I would think you could solve that by saying that Vallik has a keepsake of some kind that would reveal the location but only if you knew to look for it. A necklace made of coral that can only be found at such-and-such a reef, which doesn't necessarily seem significant unless you know that this necklace belongs to a long-time operative of Auberon. Naqualia discovers Vallik's identity in part 4, goes through his effects, and a-ha.)