1) Every skill has 2 enchant options, choose out the you like the most.
2) After choosing to enchant a skill, it displays you the success rate.
3) You normally get it by doing 3rd class change and buying them with Noblesse gatepasses.
4) Elven/Human/Dark Elven mages can enchant their skills @ Ivory, Human/Elf fighters at Aden's Fighter Guild, Orc Fighters/Mages at Rune's Temple, Dark elvens at Hardin's private academy.
5) Yes.