Also, a score isn't visible to the public until after 20 votes are registered.
A small correction - the score is invisible to the public until there are 16 votes, not 20.
The system also drops a vote if you change your vote. So if you read a story and vote on it to give it a 6 then later re-read it and give it a 7 the 6 is dropped and the 7 is used in the score calculations. I'm not sure if this process affects the total count of votes or not; I suspect that counter goes up 1 in each case and doesn't drop back down ...
No. The vote count is unaffected by recasts of previous votes. One account, one vote, one vote count.