Perhaps: Ask the driver.
1. What last request can we fulfill for you?
(offer the dead something. If they cuss us out (as I expect the cultist would), then the rest is moot in any case. If they have a request, that gives us nominal leverage.)
2. What is the password at the gate?
3. How many are inside?
4. Have you seen the statues the cultists use?
5. Where are they kept?
(I realize 4 and 5 are inefficient, but it makes a more natural flow in my mind.)
Anyways -- please offer refinements if you wish. Happy to all go at bandits inside, despite our earlier plan (which I think was made and agonized over because many of us didn't want to go in the water.).