Currently the grandma is the keeper.
After that the mom is next in line.
And after the mom it´s Elinor´s turn. So she is 2nd in line after her mother. Juliet would only come after Elinor, if Elinor doesn´t have kids by then.
Cal´s family is aware about the blood paralyzing people. The dad mentioned it during the training. And it doesn´t burn, it´s just paralysis.
I feel like Elinor was a bit vindictive in this episode. She knows that Juliet has a crush and cares for Cal and specifically pick her as the kill.
Your suggestion about just letting Juliet feed on her wouldn´t work, since it specifically has to be a kill. Or it wouldn´t count, and Juliet would still keep getting more sick.