Let be be finale of seem.
Leave it to Stevens to include a grammatical abstraction in an otherwise object- and reality-heavy poem. Here “seem” (or fantasy, the potential for the best something could be) has come to an end, becoming simply “be.” Whatever potential there was for what the woman, or the funeral, might “seem” to be is gone. There’s no more striving toward goals. What is, simply is—plain as it may be. So be it. See?
Possibly also a reference to Hamlet 1.2, where Hamlet famously (if somewhat problematically) insists on a lack of division between appearance and reality:
Seems, madam! nay it is; I know not ‘seems.'
But I have that within which passeth show…
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