Does it work for offline mode servers?

Yes.

Why does SuperbVote show %player%?

Update to SuperbVote 0.5.4 or above, which should fix all of these cases.

How do I configure a top voter sign?

Place down a sign. The first line should be [topvoter] and the second line the position to show as a number. For instance, "1" shows the top-most player, "2" shows the second-top-most, ...

Why the need for a explicit default reward?

This is mostly a matter of personal taste, and it simplifies the configuration. With an explicit reward, you can broadcast a message to players encouraging them to vote.

You do not have to run any commands if you don't want to give a reward. Just set commands: [] in the reward block.

How do I disable the vote reminder?

Set vote-reminder.on-join to false and vote-reminder.repeat to -1. See this section.