[gilbertocarlos]

1) Thrown weapons
Mountains will limit move, this means thrown weapons will be more effective. On plains, a range of 20y means that you have 4-5 seconds to hit your enemy with move 4-5. At mountains where move is halved, you have 8-10 seconds.
As for what weapons to throw, I'd say darts, spears, axes, bolas and sticks, someone very poor may throw stones. You can make a case that slings will show up if you allow to reload with a shield. My house rule says 2 extra seconds for that(And my baseline is 1(3)). Some exotic tribe may use lassos, since with slower move, there will be more chances to do it.
If they have any access to metallurgy, they should use pila and plumbatae.

2) Shields
Since there will be more thrown weapons, there will be more shields, the heavy kind if possible. A shield is one of the best options against thrown weapons.

3) Melee weapons
Swords will be even scarcer. What you should expect is people who throw spears use the spear, people who throw axes use the axe, everyone else should probably carry a knife or a long spear if possible

4) Extra skills
Parry thrown weapons will be very good for anyone who doesn't want only a shield to rely on. Tactics will be very good for ambushes, since it's easier to hide and attack from thrown weapon range. Stealth will be good to stay safe from ambushes.

5) Armor
Armor will be almost always from cloth or leather, you may get straw or wood for very poor tribes, but I doubt it. Since move is already compromised, you can see more people on the light encumbrance, warriors will be at medium encumbrance. This is partly due to armor, partly due to ammunition.

6) Nobility
If nobility uses bows, it should have a shieldman like in the bronzeage or a paviseman like in the medieval times.
Forget about horses or anything like that. Nobility will carry the best armor possible as always. I think it's more likely that it will carry bigger shields and good melee weapons, serving more as a defense unit for the rest of the band.

7)Terrain usage
Mountains will help to use cover, attacking from a higher ground will increase your weapon range, you can even use boulders from a higher ground.
The mountainous land will also help to hide better traveling bands who don't want to get ambushed or attacked.

As for your other questions
1) Slings have an advantage of ammo and range and will be very good if you allow to use them with shields.

2) They could, depending on their culture, arabs and europeans used different styles of shields even though they fought for centuries.

3) Shields would be better than buckler on most cases if they're heavy, since they are the primary method of defense. However, if you expect it to break, or to drop it, I strongly suggest using bucklers.