Like Belial said, Aleta is a special case. Her immortality has to do with her identity, not with her (former) position as a CoV leader. Her imprisonment by Vasir has been going on for a while and doesn't look to end anytime soon, leaving her indisposed to returning to the Cult.
As for how difficult it was for Rita to kill Candit, well, that's unclear. Candit's less about physical combat and violence as she is about powerful spells--it's possible that Rita was able to overpower her by surprise. Without time to prepare, Candit wouldn't be much good. With time, she's much more formidable.
It's also unclear how long the struggle for power lasted. Was it a brief fight that lasted as soon as Aleta was captured (or soon after, before Candit suspected betrayal)? Did Rita invite Candit to a meeting to discuss the new leadership structure, perhaps offering to divide power between the two of them, and then attack? Like so much, especially Reish-side, the lore's unclear.