Wow! There are some really excellent points being made here. I *love* the dual interpretation of Ennis affixing the numbers to the mailbox - that's brilliant, and like Amanda points out, ambiguous, probably intentionally. I see dual interpretations and ambiguity pretty much everywhere I look, though. And I change my mind, depending on how I'm feeling when I watch.
I tend to think Ennis found some measure of peace at the end. To me, the shrine of the postcard, the bloody shirts (reversed!), and 'I swear' combine to tell me that Ennis has come to terms with who he is and with what he and Jack meant to each other. His willingness to skip the roundup and attend Junior's wedding supports this - Ennis has come to understand what it is to love. He can directly relate to what a wedding symbolizes (He had no clue when he himself was 19.). Further, he appreciates what it means that Junior is able to celebrate her love openly - an experience he and Jack were not destined to have.
Keeping the shrine in the closet (movie) alludes to gay people having to hide their identity (obviously), but people also keep their most precious possessions hidden in safe places. Keeping a treasured memory private gives it a power that may dissipate when it's shared with the world. Again - a dual interpretation.
I feel the same way about the open window...there's the obvious door shutting/window opening interpretation. When we see the wind across the field, we think of Jack. Ennis is still in the trailer, but at least he is looking out that window. Regret? Certainly. Future? Possibly.
Obviously there's a great deal of sadness throughout Ennis' whole life - he tried not to be a 'sad daddy' with his daughters. But somehow I don't see depression at the end, in the way we usually think of depression. There's certainly some apathy with respect to his physical comfort: 'Don't have nothing, don't need nothing.' But is it really apathy? or contentment in a minimalist sense? He's lost what was most precious to him, so everything else necessarily pales in comparison.
In fact I think that is why Mrs. Twist does put her hand to her throat, not because of Mr. Twist but because in looking into Ennis's eyes after he finds the shirts she now understands everything between Jack and Ennis, and she feels Ennis's true grief and she is feeling the pain of his pain.
I completely agree that Jack's mother understands the depth of Ennis' grief in Lightning Flat. To me, her hand on her throat is a physical manifestation of this understanding - that the depth of her (and Ennis') pain cannot be articulated. Ennis only manages a respectful nod and a mumbled 'Thank you for this', indicating the shirts. But his eyes convey his gratitude more than any words could ever manage. Alternate interpretation: living with OMT has caused her to gauge her words carefully. Hell, there's a kitchen scale on the counter - they live a measured existence! Mrs. Twist may want to contradict her husband about the ashes but doesn't dare.
(Another question, why does he now have a fancy knife set, when before he could barely even eat? I have been wondering about this.)
I've thought and thought and I have no idea what the knives mean. At least they're real knives instead of electric ones! Perhaps we're meant to remember Jack's confrontation with L.D. - a tribute to Jack finding his voice and standing up to his father-in-law vs. Monroe's milquetoast reaction to confrontation in his home. Is there a possibility that Ennis may find his own voice?