I will try to answer all questions as good as possible:
1. When should I feel safe about liberating/suppressing the cities?
- Whenever you want. Just beware that if you want to follow Beliar, you have to get 75% with Hashshin, and if you want to follow Innos you can't support the orcs as well.
2. Do I need to liberate/suppress all cities to get the outmost ending for the side I have picked?
- Nope. You can finish the game without liberating/suppressing any city at all. In most cases this is done to get reputation, loot and xp. The most valuable is reputation, but there is no need to help out all cities.
3. Do I need to find the fifth artifact for the "best" ending?
- You need it for all endings.
4. Do I need to talk to the leader of the Hashishin (Zubar?) for a "good" ending?
- No, but you certainly need to do something else to him. No need to get into Ishtar the non-violent way though.
5. Do I need to talk to the leader of the Orcs (in Faring) for a "good" ending?
- Not at all. You can complete the game with 0 reputation with orcs. In fact, if you are roleplaying a very good character you probably should keep it as low as possible, although it won't affect the ending itself.
6. Do I need to talk to the leader of the Nomads for a "good" ending?
- Technically, yes, as the leader of the Nomads is Saturas, and he helps you to get one of the artifacts. Other than that quest, there is no need to do anything more for the nomads for the good ending. You can also kill him, take his temple key, and still complete the game the good way.
7. Where is the leader of the Nomads?
- It's the water mage Saturas, located in the ruins south east of Mora Sul.
8. Is there a nomad uprising in Ben Erai, in that case who leads it?
- You can get one of the guards there, a named one standing near Sanches, to start a rebellion. He won't do it for the nomads though, but for gold.
9. Is there a nomad uprising in Mora Sul, in that case who leads it?
- A fellow named Kyrau or something, he is located in a cave north west of Mora Sul, along the mountains there.
10. Is there a rebel uprising in Faring, in that case who leads it?
- Nope, as the capitol city of the orcs, you have to face that one alone. Same goes for Ishtar in the desert. These cities hold the faction leader and you'll be given no help during the assault.
I hope that answers your questions without spoiling too much. Good luck!