How could we identify the similarities between Ethan Frome and Willy Loman as tragic heros?
I will answer your question based on traits in order not to discuss both characters separately in too much detail (which would be two different questions).
The trait of the tragic hero involves a series of events that occur to a character who inevitably has to deal with them. There is no outside control nor help from fate that could prevent a tragic hero from a sad ending. It simply is as if destiny has chosen to make the character suffer because of a tragic decision or a tragic flaw.
This being said, Ethan Frome would perhaps fit the tragic hero characteristics better than Willy Loman. This is because the tragedy of Ethan Frome is his inability to change his life from a tragic flaw. He has a series of tragic obstacles that include his diminished financial resources, the influence of his wife, his lack of freedom, the intense emotion he feels for Mattie, and, finally, his accident, which leaves him disabled.
Ethan does not have many options to turn his life around, nor to start over in a different direction. His biggest tragedy is that he knows that he has a dream, and that he cannot make it happen no matter how hard he tries. The accident that he suffers in the end is the best evidence of this: He could not even complete a suicide- fate wants him to live for the rest of his life in complete and abject misery.
Contrastingly, Willie Loman at all times has a chance to give up his dream and go with what he really loves. He has signs all over the place telling him that he is chasing waterfalls. He is well in charge in his marriage and even takes in a mistress just to justify his egotism. Willy got what he got because he basically built his path towards a sad ending, not because something is hovering about his existence but because of tragic decisions.
Therefore, metaphysically speaking, Ethan Frome may the true tragic hero from a flaw, whereas Willy is also a tragic hero but from decisions.