How convenient that both the baron and the duke are away. I trust Ellie made some other preparations. If the duke or baron had a priest they trusted personally, such a person being present would be a very good thing, as the (fake) priestess couldn't then bullshit anything about the temple without getting caught lying. The trusted priest would also recognise if there's something wrong about the holy water. Of course Ellie could just have another bottle arranged and tell the (fake) priestess they will be using that one, not whatever the (fake) priestess is bringing with her.
At least they won't be among random people this time.