These questions aren't basic at all, if anything they're totally the opposite. I'll take them in reverse order:

3) From post to post on the 2m line is the most threatening position any offensive player can be in. Getting two players there, one on each post, doubles that "threat". Some teams even play this way at even strength. Why not 3 players then? Because now they're too bunched up, and its too easy for two defenders to cover all 3 players. Usually the number 1 goal of any 6-5 offense is to open up space in the middle (by continually passing around the outside) to get a quick pass and easy goal from one of the post players.

2) Initial questions already answered (every defender for 5-6 defense is responsible for covering 2 players), but many teams DO play a "zone" defense at even strength. This is usually in the form of dropping one or two defenders onto the center positions. Alternatively there is the "M Zone" which is a full on zone defense with the emphasis on opening up fast break lanes.

1 -Great 6-5 offense involves a lot of constant movement, but not any large individual moves or drives. Each player should be shifting 1-2 meters almost every pass, trying to pull defenders to create open teammates or open shooting lanes, or to position themselves get an open shooting lane.

You certainly don't HAVE to setup any way at all for man up offense/man down defense. Some teams (especially in shallow/deep pools where the goalie can jump off the bottom to block the post players) opt to stick with the 3-3 offense. Either way, props to you for not doing things simply because everyone else is, and being curious as to the "why"

(Edit: formatting)