Continuity mistake: When Krell jumps out of the tower window, his blue lightsaber is in his right hand, but when he lands, his sabers have swapped hands, with the green one in his right instead. (00:12:21)
Continuity mistake: Tup attracts Krell's attention by firing two shots at him, but Krell is then seen deflecting three blaster bolts. (00:15:41)
Continuity mistake: When Rex releases Fives and Jesse from the brig, Fives' kama is missing as he stands up. (00:11:00)
Continuity mistake: When Rex begins to question Krell in the brig, Fives' kama is missing. (00:16:55)
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.