I agree with your latter point, Dread but only to an extent. I greatly enjoyed the film, but yes, the ending felt off. I immediately wondered "What was the intent? What is one supposed to take from this? That one should stick to his convictions no matter how insane he feels/seems? Or that he should let come what may? Because the chances of them surviving are none. They were at Myrtle Beach but they lived in Ohio, correct?
That is why it still works. Because the point is not option A or option B, it is decision. He makes a choice and at least in the mortal sense, it is the wrong one. But at the moment, in the shed, it is the right one for the sake of his family and his sanity. Life is uncertain. You could hide, planning for events that may never come to pass, at the sacrifice of those close to you and most importantly yourself. Or you could live. Even in doom, he choose life and is redeemed by the arrival of the storm. That final word from Chastain: "Okay." captures this all.
It is also important to understand that a protagonist is allowed to make a flawed choice. We are just so used to her making the 'right' one. The real 'right' choice is the true choice and that is always the human one and Shannon's character makes his human choice.