I would like to comment on some things:
1. No. Unless you count the satanist lair.
2. No.
3. I think they were supposed to be used for something. I haven't checked yet.
4. I think she was supposed to, but I haven't checked.
5. I don't think there is a level cap, but I don't know. Haven't seen any code about a limit, though.
6. You mean the last that get available for purchase? Yes.
7. I have never smuggled, and haven't checked the code for that.
8. I think you are supposed to somehow be able to fight them and keep the money.
9. Haven't checked.
10. I don't know about this.
11. It seems he is supposed to be able to give information about Raoul Rheims, possibly if you play as Danielle.
I know the Build Mod changes some of these and enables some things, but I don't know about their exact status in the stock game. I will check some time.
I can attest that the Xbox version is unstable, at least the American version. The "white screen of death" is particularly "hilarious" - the entire screen gets white and the game freezes, it's like the game has surrendered. It also gets a lot of "bad saves" for no apparent reason. It can also crash in other ways. There are some workarounds for some of these problems, but they don't always work.
If the Xbox is modded the games can also be modded, but I guess you have an unmodded Xbox. I have made some progress with modding Pirates of the Caribbean on Xbox, but haven't tried getting a Build Mod version to work on it yet. I intend to work further on this.
The PC version is better, so you might want to get it. It should be possible to get a controller working if you prefer that.