Crossword puzzle clue "last letter in most plurals but not in this puzzle's six longest answers which are the only plurals in this grid"

Last letter in most plurals (but not in this puzzle's six longest answers, which are the only plurals in this grid)

Letter that begins and ends this puzzle's six longest answers

Three-letter combinations hidden in this puzzle's six other longest answers

Letter appearing only in down answers; its opposite appears only in across answers

Campaign about which Rumsfeld said in 2003 "it could last six days, six weeks - I doubt six months"

Angle irons graphically represented by four sets of black squares in this grid, and by letter formations starting in the four longest answers

The 2x2 black square near the middle of this puzzle's grid, e.g., which is part of eight answers

Number of times this puzzle's theme word appears diagonally in the grid (to complete the puzzle, shade those words plus the Down answers that begin where they meet)