You seem to be misunderstanding how diceware works. You randomly generate numbers by throwing dice. Every five rolls indexes exactly one "diceware" word. So even if an attacker knew we were using diceware, each word contains

    log2(6^5) = log2(7776) ≈ 12.9 bits

of entropy. If you want 128 bits of etropy in your security question field, then just randomly generate 10 diceware words. This is comparable to choosing 20 random printable ascii characters or so.

Since we pick the words by literally throwing dice, English grammar has nothing to do with it.