According to the "Golden Rule" on pg. 11 of the rule book, unless the word "cannot" (and I assume "can't") is present in a rule a card's effect takes precedence. With that in mind, would "A stinging insult" then force an opponent's unit that has one or more focus tokens to defend? As the declare defenders rule does not stat that focused units cannot defend, I would assume so.

Example: On my opponent's turn he attacked with Emperor Palpatine. On my turn I play "A stinging insult" targeting Palps to force him into an engagement where I can "safely" damage him.

The text on "Stinging Insult": " ... That unit must defend this engagement, if able."

In this case, since Palpatine is focused, it is not able to defend in an engagement, so it will not do so.

The rule book says that the defending player declares which of his ready units will engage. A focused unit is exhausted not ready, so I would think that no Stinging Insult, which states "if able," does not force a focused unit to defend.