This is the least what my knowledge about spriting can give you... xD (thanks to Sir DevilEvil's Tutorials )
1. You can copy an official RO job sprites then paste it to your custom RO job folder, then using actOR replace it by "Saving as or just Save".
2. Maybe the sprites are the same but the .act files not. There are act files for each jobs.
3. Use an official RO Designs as a body Reference.
4. Yes
Thanks... xD