I think you're right to see Zosima and Ivan as two sides of the same coin rather than as complete opposites. They both share a pretty bleak view of society, both have a fatalist's assurance, and both are merely bidding their time until the great change in the future. Zosima waits in the monastery cultivating Christ's image, and Ivan plunges himself in "Karamazoving" (I think we all know what that is). Does the novel agree with these characters? Partly, yes. Their indictments of then contemporary Russia seem to hold up. The novel doesn't appear to correct their observations about the suffering and cruelty going on around them. Yet I don't think the novel supports their conclusions about fate and direction of society. The book ends with neither a great Christian overthrow of society, nor an authoritarian church being set up. It ends with Alyosha's speech which embraces the flawed life that Zosima and Ivan are forever running away from. Alyosha tells the children not be afraid of life, but rather to pursue the community and love and preserve that image (not Christ's directly) in one's mind as a protection against cruelty (that within and that without). I suppose you could recuperate Ivan and Zosima's interpretation by saying that the end of the novel is not really the end of the story, but I think that's a bit of stretch. The real conclusion seems to refute their revolutionary mindset, and instead support Alyosha's attention to life.
I tend to disagree. You propose that last sentence as though it were a counter-argument against Ivan and Zosima, but it sounds more like you're agreeing with them. It's Ivan Grand Inquisitor who builds his entire argument on earthly limitations: "Then we shall give them a quiet, reconciled happiness, the happiness of feeble creatures, such as they were created" (337). I think the point of the novel is get beyond this. Otherwise, we'd be left with a depressing (and not to say banal) outlook on society. There would be an elect few who aren't tainted by society and then everyone else who are doomed to live in either anarchy or authoritarianism. Now that's uplifting! Again, I have to stress that Zosima's and Ivan's philosophy doesn't seem to match the rest of the novel. While it's true that Aloysha is childlike and innocent in many ways, he's also the character that warns people from being afraid of life--he's also the character that tells Ivan he isn't guilty when the Zosima/Ivan philosophy tells us they are (and Ivan and Dmitri get away!). Alyosha's also the character who emphasizes lived acts of kindness and not an abstract Jesus-image like Zosima. The characters who shelter themselves from perceived societal evils are not exactly role models in the novel. Zosima waits in the monestary, and doesn't do terribly much for us in the book. Ivan, Dmitri, and Fyodor hide themselves in "Karamazoving," but that doesn't go particularly well for any of them. That's why I wouldn't necessarily say the message of the book is to ware hairshirt and control our urges. Yes, Alyosha is above his impulses and the others are not, but what's pointed out several times is that characters resort to sensualism because they don't see any way forward with society. It's not that Ivan tells Aloysha that he's "Karamazoving" because he's just not above his impulses. Rather, he's "Karamazoving" because of the views he expounds about society. Ivan's worldview excludes the possibility of productive labor, meaningful kindness, and love. So, why not drown himself in sensualism? That's what he tells Alyosha in Pro and Contra, and that's what's reiterated in the novel.

