I’m absolutely happy with the z=2 value.
If no one else disagrees, then I’d say we have a consensus here:
Rank the questions on (upvotes+2)/(upvotes+downvotes+4).
It’s an easy formula (therefore transparent even to people with no mathematical knowledge), and at the same time has a scientific basis (the Wilson score interval).
The question that remains is what to display on the answers. My suggestion would be to simply show the number of upvotes and downvotes separately.
Note: If we decide on giving different weights on votes from differently experienced people, then upvotes/downvotes here should refer to the sum of the weights of the corresponding votes.