About the issue of the "central tragedy" in the film... Well, clearly everyone will see this differently, but to me it comes down to lost time.
By not living together they lost vast amounts of time together and, clearly, once Jack was dead the time is lost permanently. So, now Ennis's motto, "If you can't fix it, you have to stand it" becomes his "life sentence" so to speak. The tragedy for me is that they didn't fix it while they could. And in the end Ennis has to "stand" living without Jack for the rest of his life now that the chance of fixing it has passed him by.
And, as I posted above, the idea that they went to such efforts to live apart to appease a homophobic society (or even the homophobic society in Ennis's head) is a big component of that central tragedy. In an ideal world, honoring their love for one another should have taken priority over living fake lives to conform to societal pressures. Conforming to those pressures compounded the tragedy because it then impacted the women in their lives.