Please understand me, I don't think there was anyway Ennis could just walk away from Jack, in the deepest part of their relationship, it wasn't about being gay or straight or what it meant. No matter what Ennis said or did, proclaimed, bore witness to, etc. Ennis couldn't just walk away. To walk away from Jack, would be an ending to his own life. His life was Jack, unknowingly, knowingly how ever you want to look at it.
Ennis was not philosophical guy, he existed on his need and love for Jack, no matter how badly he treated him. If Jack would have just turned away from him, there is no way Ennis could have handled it.
He wrote to Jack confirming November. He did something out of the ordinary, he wrote to Jack to make sure he was coming, how long was it that he might not have heard from him? (forget Jack was dead, let say he hadn't died), How crazy do you think this might have made him? What would he have done?
IMHO:
He would have done something, he couldn't just let it go, I don't think he would have ever let Jack go, if he couldn't have him I don't think anyone could.
I know that may be an ugly way to look at it, Ennis was not a saint, Ennis could be brought to violence, when it came to Jack he could fall to his knees, or raise his hand. But to just turn and walk away, when it came to Jack I don't think Ennis could or would, he would have done it 20 years before.