Continuity mistake: When Fives is telling Rex he, Jesse and Hardcase are going through with their plan, Jesse and Hardcase switch places. (00:12:26)
Character mistake: When Jesse says that he's "really not liking this", Fives calls him Hardcase when he responds with "don't get an itchy trigger finger." (00:15:35)
Continuity mistake: When Krell is getting in Fives' face about who is going to take the blame for Fives and Jesse disobeying orders, the markings on Jesse's armour are missing. (00:21:21)
Answer: Dooku never accepted Ventress as his apprentice, as far as I know; he just kept her as his devoted personal assassin. In Phantom Menace, it is stated that there can be only two Sith at a time, to prevent a power struggle in the lower ranks. To accept Ventress as an apprentice, Dooku would have violated that rule.