Wave 1 - USS Defiant - USS Defiant - Crit Conversion

1. Can the U.S.S. Defiant convert crits for all attacks against it per round or just for one attack against it per round (my choice or first attack?)?
2. If the opponent uses Toreth as the captain she can convert one hit back to a crit after I used the Defiant's ability?
2. If the opponent uses Toreth as the captain she can convert one hit back to a crit after I used the Defiant's ability?