I would love to jump into many scenes, not just one.
I would go way back into the past and shoot the people that killed Earl, just so that Ennis won't be corrupted, and Earl and Rich might have a better shot at life together. If that's too late, next.
The Brokeback incident was too beautiful to interfere with so I would leave them alone up there. What I would do is to convince Ennis to ask Jack for a ride after they come down the mountain, or convince Jack to give Ennis a ride and they drove to Lightning Flat instead. Too late? Next.
Convince Ennis not to get marry, and convince Jack not to get to know Laureen. Failed? Next.
After the divorce scene, convince Ennis not to send Jack away, but wink at him and tell him to wait in cabin till evening. Then Ennis can have fun with his girls, bring them home to their mother afterward, then spend the rest of the week with Jack. I agree that Ennis need to be kick in the ar** sometimes to wake him up.
In the final meeting, make Ennis hold Jack after he said,"Sometime I miss you so much, I can hardly stand it". Then tell them they have to stay together to survive.
In Jack's death scene, I would shoot, stab, or cut-up his attackers thus giving Jack another chance. Then telling him he's not safe here in Texas anymore and he has to see Ennis. But if I'm to late in saving him, I would call an ambulance and stay with Jack to make sure he won't drown in his own blood. If he's dying, I would be there holding his bloody, broken body so he would know someone is with him.
In his scene with Jack's parents, I would create some diversions to get the Twists out of the house so that Ennis could take some of Jack ashes to be scattered in BBM. If I failed that....
I would hug and cry with Ennis in the final scene.
If I have to choose one, it would be Jack's death scene. If he's alive, there will always be chances.