And I'm not sure whether it's simply a fear of the unknown that makes Riku's darkness. Part of me thinks his darkness comes more from a fear of not having a place in life.
Well, not having a place in life certainly becomes one of the main catalysts of his darkness, but it isn't the original one.
In BbS, he says he wants to leave to find strength. If he really thinks that he must protect the things that matter, then there's obviously something in his mind that he needs to protect them from (something from the outside). And that's where his role as the protector comes in, and he clearly thought of himself as that the entire time.
But his notion is shattered when Sora gets the Keyblade. Only then, I'd imagine, does he feel like he lost his place in life.
I think it was more that he was hurt and shocked over having the mantle stolen from him, and over time built up all the reasons you stated in order to justify why he deserved that role more. And he still tried to play the role of protector, going to extreme lengths for Kairi, struggling along without the support of the Keyblade and not trusting Sora to get the job done right. I don't think he consciously passed that mantle on until the very end, with the line 'take care of Kairi for me'.
Perhaps "passing the mantle" was the wrong phrase to use their because it entails a willing passage, but I'm in agreement.
What I meant there was that, when he saw Sora with the keyblade, he saw him for the first time in the role that he once thought was exclusively his. He applied that role to Sora, and judged him from that point of view (and rationalizes that Sora isn't fit for it when he sees what happened to Kairi).
As I said, Terra promised Riku the keyblade ("So long as you have the makings, then through this simple act of taking, its wielder you shall one day be."), so there's no doubt that Riku felt robbed- both in terms of the material and his role in life.