I have only played G2 and G3, but this is my opinion:

1.) I think it is both, but in my opinion it should be an Ore daemon, because the Ore is magical and I think it was the guard of the ore or a daemon that was attracted by the ore, so that it got its name from the ore. Also the common name for the sleeper is in German: "Schläfer" what is the direct translation for sleeper.

2.) Yes.

3.) There is no clear good or evil. Just let us talk about Innos. Innos is the god of order that punishes those that offend the order with fire. So stealing is against the order but what happens if someones has to less money to pay the food and have to steal it? In a world ruled completely by Innos this guy will be prisoned or even killed. Do you think this is what a good god would stand for?

In the other hand Beliar is the god of the live after death. Let us say that someone you know dies, would you really think that the death is good, just because this is the natural order?

The point is that there is no real good or evil, like in the real world. I think this was the message behind the 3 gods.

In gothic 3 you learn that the player itself is a chosen of Adannos (the neutral god and the creator of the whole world). To be the chosen one means that you arent the servant of any of the gods at all, so that you are completely free to choose the god you will serve.

But if Xardas is as much a servant as the player of any of the gods, this could mean that Xardas is also a chosen one of Adannos that isnt influenced by any god, but can freely choose which gods power he is using. Another hint for this theory is that he was first a servant of Innos and later on started to serve Beliar while he seem finally to serve Adannos as implied at the end of Gothic 3.

4.) Maybe the book is talking about the orcs as the evil which attacked the kingdom. Those orcs couldnt leave so they couldnt bring back the ore to their comrades and use it against the kingdom.

5.) The orcs were controlled by the god Beliar (maybe Ur-Shak wasnt affected, like the hero or xardas werent affected by any god). At the end Xardas collected the essence of Beliar and became immediately something like a god that gained control over the orcs. This changed many things also it is mentioned that there exist many Orc tribes, some of them are more human like and others are more animal like.

6.) To get strong enough to be able to deal with the chosen one of Innos, the king (his own father as I understood it), maybe because Beliar thinks that the anger (of being prisoned in the colony for nothing) about the king is big enough to do so.

Another answer could be that Beliar is the god that controls the death and his servants will come to him, also I doubt that he really cares about the death at all.

I just finished replaying 1 2 and 3 a few weeks ago, so I'll give you my opinions. I've only played the game in English, so it's very interesting to learn what you've said about the German word Erzdämon. I would say any of the interpretations you've thought of could be true. The Sleeper is a very powerful demon, so he's an archdemon, but he also may have some connection to the ore itself. The ore is only supposed to exist in the valley (until Gothic 3 when it suddenly exists in many different places), and the miners sometimes say things about how the ore seems to have a mind of its own and resists being taken out of the ground.

Remember that The Sleeper actually has the power to make people fall asleep too (he makes Xardas fall asleep, for instance), which is yet another interesting factor. I don't remember for sure whether The Sleeper is ever said to be an avatar of Beliar the way the dragon is, but it's possible. I'm not sure Beliar is exclusively evil, just like Innos is not exclusively good. There are some non-evil characters in the story who have shrines to Beliar, like the witch who lives in the cave behind one of the farms in Gothic 2. He's definitely a god of death, destruction, and chaos, but he also seems to be strongly associated with magic and knowledge, which is why Xardas and the witch follow him.

Once you get to the end of the story, you see that Xardas has been trying to use Beliar's power to try to get rid of all the gods, so I don't think you can say Xardas ever really shares Beliar's goals. The mages didn't know The Sleeper was imprisoned in the valley before they created the barrier, because it was the orcs who had summoned and then imprisoned him a long, long time ago. It was the powerful magical presence of The Sleeper that made the barrier spell get out of control. Because they're irredeemably bloodthirsty and savage. Lots of things in G3 don't make sense. Orcs in G3 are Klingons. I think the dragon's whole plan was simply to draw the player character to his lair. The main character, like the dragon, is a chosen avatar, so if the dragon could kill the main character, that would be a huge victory. There's no better place for one avatar to fight another avatar than in his own lair.

3.There is no clear indication of Innos punishing people with fire if they do something Innos, if people do something bad they are just punished by the law accordingly.Innos would care for fair trials and judgements !
There is actually evidence of Beliar bringing punishment . Looks here in WORDS OF THE GODS,, volume 1(Gothic 1) "The word of Beliar: But whosoever shall do wrong and go against the will of the gods, him I will punish. I will plague his body with pain, suffering and death, but his spirit shall join me in the shadows forever more.".
Beliar basically says that he will punish you if you go against the will of the gods, bring you death and disease and that you will go to his realm as a ultimate punishment. And his realm is the Gothic version of hell ! So yeah, bringing death and disease for something like that is pretty evil. So yeah, there is real good and evil with these gods, like in human society !
" In the other hand Beliar is the god of the live after death. Let us say that someone you know dies, would you really think that the death is good, just because this is the natural order?" That's just factually wrong, there is no proof or source which says that Beliar is the god of life after death ! There is only this passage "But whosoever shall do wrong and go against the will of the gods, him I will punish......... but his spirit shall join me in the shadows forever more", so Belair basically says that you go to his realm ONLY if you go against the will of the gods, it's not a standard afterlife !
Also, there is a misunderstanding of people going to Beliar's realm after they die on the OATH of the water magicians ! But they don\t say that you "SOUL" goes to Beliar's realm, they only say that your body goes to Beliar's realm, which is linked to the fact that Morgrad(the earth) is at the heart of Beliar's realm.So, ended up misreading this and taking it as lore !
So, to summarize, you only go to Beliar's realm if you basically go against the word of the gods ! And his realm is not a actual spirit realm, there is only evidence to the contrary, it's just a dark place linked to the material world and a place most of his followers go.

Also, Adanos is not a "morally neutral God", he just makes sure the battle between Beliar and Innos(which was started by Beliar", doesn't destroy the world ! That counts as being a benevolent god ! While Beliar is the one who started all this mess and doesn\t really care about the well-being of the world, meaning that he ain't benevolent in his actions.

4.I don't remember what that almanac book says, but the G1 intro movie has Xardas's voice saying that " something distubed the delicate structure of magic and made the barrier much larger".The mages, and not even Xardas, expected the barrier to get so big, and it became bigger cuz of the SLEEPER, he is the one who disturbed the structure of magic and made this happen. 4.I don't remember what that almanac book says, but the G1 intro movie has Xardas's voice saying that " something distubed the delicate structure of magic and made the barrier much larger".The mages, and not even Xardas, expected the barrier to get so big, and it became bigger cuz of the SLEEPER,you find out THE SLEEPER is the one who disturbed the structure of magic and made this happen. Xardas actually started to study the black arts to find a way to bring down the barrier, so they barrier being bigger was not caused by Xardas, but by the Sleeper.