Hello francis. I must admit that I had not thought of it this way, although I love your second interpretation very much.
On a prosaic level, Ennis looks out the door to see if Alma Jr. was already beyond calling back to retieve the sweater that she left behind. I must admit, though, that in the loneliness of the final shots inside the trailer, I did, in my mind's eye, try to see Jack as with Ennis. The shirt is the proxy. Jack is indeed no longer there, but his love is alive in Ennis.
This is how I interpret the "presence" effect in the last scene. Jack is there in the form of his love for Ennis. The shirt is the material reminder. And Ennis will be the sanctuary.
So long as Ennis breathes, Jack lives.