Based on the options offered by TheBombzen's original AutoSwitch mod. These include:

• Enable by default: This option determines whether to enable AutoSwitch on new worlds and on worlds AutoSwitch hasn't been used on before.
• Use in creative: Use AutoSwitch when in creative mode.
• Use on blocks: Use AutoSwitch when digging blocks.
• Unswitch on blocks: Switch back to previous item after digging a block.
• Use on mobs: Use AutoSwitch when attacking mobs.
• Unswitch on mobs: Switch back to previous item after attacking a mob.

Option 1 and 2 should be simple ON/OFF toggles, while option 3-6 would have:

• Never
• Always
• Singleplayer Only
• Multiplayer Only

• Enable by default: Not sure on: how to implement, it's usefulness
• Use in creative: Implemented, will be in 0.5.0
• Use on blocks: Implemented, will be in 0.5.0
• Use on mobs: Implemented, will be in 0.5.0
• Unswitch on mobs/blocks: Planned, but unsure how to implement

EDIT: Is separating those options based on SP or MP state really all that useful?

MP check added as it's own thing.
As for helping, a lot of the logic for AS is just java, not MC specific. Feel free to make a PR or suggest things.

OK, nice. Yeah, I'm not that sure on the first one too, but it may be useful to some. Unswitching on the other hand is really important, I hope you can figure it out. I'd love to help, unfortunately I'm still not that experienced in Minecraft modding.

While I personally use AutoSwitch in both SP and MP, many consider it cheating and some servers even ban you for using it. So it's definitely important to have this option, yeah.

Version 0.6+ will have switchback.
Other options were addressed earlier. As I see no need to separate the functionality of each option by SP/MP, and no need for checking if AS has been used on a world previously, this is closed until I am convinced otherwise.