1) Each of them is helpful to a different role. If you think you'll be playing a lot of classes, Leader is always a nice pick - everyone can use some extra HP and MP. If you're planning on taking thief, boosts to AGI, STR, physical attack, attack speed, crits, etc are probably your best choice.
2) Wipe after closed beta, although there are some nice prizes that players will get after the reset if they hit high enough levels. Open Beta won't be wiped, from what I understand.
3) Like everyone's said, 0-9 for the first row, Ctrl+0-9 for the 2nd, and Alt+0-9 for the third. You can change the bindings in the settings. Also a helpful tip: Use the F-keys to target party members. (F1 for yourself)