That Riku seemed more interested in stopping Ansem at all cost seems to be the case indeed, as he implores Kairi, Donald and Goofy to flee, indicating that he doesn't expect himself to be able to hold Ansem off for long.
This is all part of the whole mystery that surrounds the "Terra/Xehanort/Eraqus"-mess that this whole heart-conglomerate is and which Nomura refused to delve further into yet (even worse, in DDD he even deepened the mystery). We do not even know why Xehanort's attempt to expel Terra's heart didn't work, Ansem succeeded with Riku just fine and he had no Keyblade at hand, unlike Xehanort.
It has been theorized that this may have already been an instance of Terra and Eraqus working together against MX, but we don't know anything for sure.
I'm more inclined to believe that during that fight, Terra inside Xehanort was finally able to overcome his feelings of despair and rage to a certain extent, and seeing Aqua believing in him and trying to best Xehanort brought him into action, in the hope of not losing Aqua after he already (supposedly) lost Ven (last thing Terra saw was the energy pillar of Vanitas initating X-blade sorcery). The Lingering Will came already into being due to Terra's immense rage and determination, something we'll notice Riku wasn't doing, not because he might be incapable of it, no, at least at first Riku welcomed Ansem as an opporunity for power, while Terra rejected Xehanort from the very beginning.
I could imagine that there was at least partly an interdependency at work, with the negative emotions increasing the darkness more while the darkness caused even those emotions to intensify until Riku was incapable of any rational judgement.
Did I ever imply Aqua didn't want to help him? As you said above, unlike Sora, she was incapable of doing so during the Keyblade Graveyard events because of Braig and Vanitas. Xehanort freezing Ventus into a popsicle had certainly more meaning than just to make Terra mad and hurt him "for the lulz". Xehanort knew that both Aqua and Terra were very protective towards Ven and by putting him in peril (and send Vanitas after for the X-blade) he effectively put Aqua into a tough choice-situation. Go help Terra against Xehanort? Nope, bad idea, then Vanitas would get to a helpless, frozen Ven.
The fact that Aqua actually choose to help Ven first (she also didn't expect Braig appearing to stall her) proved that she had still faith in Terra, which she also rubs right into Braig's face when he shows up.
Being a master of manipulation though, Xehanort expected that and proceeded to drive Terra to a point where he would lose control of his darkness.
Looks like we got a spectator, *ggg*.
Of course, despite all similarities, they need some differences in order to better show off the different possible paths and actions they take.
Sora and Ventus are in fact also different in some aspects.
I would be cautious with saying Riku cheating with Darkness to kidnap Roxas and set him up to disappear against his will were the "right reasons" though. In the very end, his goal might have been noble (to awaken Sora), but not his actions. In regards to Roxas he, Naminé and DiZ weren't any better than the Organisation in that they were just using and treating Roxas as a tool to further their own goals instead of treating him as an individual with his own wishes and will.
Riku is definitely the deuteragonist, meaning the second most important character of the series after Sora. All the trios and Mickey, Goofy and co. might be main characters, but Sora and Riku still stand one level above them.
