2. There isn't a set amount, and it depends of the Perception of the person you are trying to steal from. With 200% skill, you will usually succeed.
3. Sneak is not necessary and it does not give bonus.
2. Even with a skill well over 200 you will fail quite regularly.
When it comes to npc that means:
- you lose protection, nothing more. You can only find out by keeping an eye on the town reputation
- you'll get shot. Off course you also lose reputation. Look at it from the bright side: at least you're not unprotected when you return
3. Only "bonus" it gives you is that you might be noticed a bit later than none sneak thieves. So victims with low perception have less time to react which gives you more time to steal which in return is convenient with stacked items. Off course that's best used around people who are trading. High sneak also gives you a bigger chance of getting away when you're caught stealing. Players can't shoot what they can't see.