This was a straightforward puzzle finished on Saturday afternoon (no need to burn the midnight oil or even carry the puzzle over into Sunday or Monday, as I had to do with some recent puzzles). The preamble was generous - there was no need to mention that the two words missing from the quotation were used in clues. If I'd been setting this puzzle, I'd have had the two clues where the missing words appear (with some minor tweaking) as wordplay only, leaving solvers to work out that the definitions were the two missing words.
Fortunately, I'd started with the later down clues, so identified the author quickly and a look at the Wikipedia article for the author gave the poem right away.
As regards the puzzle title, I wonder whether we are supposed to read this thematically as (P)revision - the poem could be regarded as a prevision of the author's death during the Battle of the Somme on 4 July 1916.