Gday!
1) The portal slab is not related to the teleporter stones in any way. You may have noticed the door beyond (to the left) of the portal slab is locked from the inside. The portal slab is there so the player doesn't get locked/trapped into that particular area if the player teleports to the second teleporter stone while carrying the first (given by Lanilor). That area can alternatively be accessed as part of the normal progression through the dungeon under the first town, but it helps alot with healing if you have both stones before starting and place 1 next to the bed in the room where you start. Just don't try to take out the guys in the room if you do teleport into it, there way too powerful.
In order to use the teleport stones, one must be on the ground, while the other can be either on the ground or in your inventory. If a teleporter stone is in your inventory, and the other on the ground, you can only use the one in your inventory to teleport back to the fist, but now you have no way of teleporting back because your carrying the second one.
Thats a little confusing isn't it? Just play around with them to get the hang of them. Usually you place 1 next to the bed (e.g. the one where you start the game), and when required, drop the second one, use it, rest, return, and pick it up and continue on. I use to have panic attacks playing the game on whether I had remembered to pick the stone up after returning from resting.
2) Yes, it is a range, and as you said, adding the two numbers gives the maximum range value, while the first value is the minimum.
3) Check the well just to the east of Madaneus's house. Some quests do require points in the lockpick skill, although you can find/purchase items that grant +1 to particular skills. You do not need to assign any points in the lock pick skill to finish the game.
Good luck!
-Fabian