Please look here http://www.dudleyville.com/bg1/ar5404.htm
(it is inside the cave --- so you have to find it from the inside). And fortunately, you walked back the way you came. This means that you are ablo to go through the levels of the Nashkell Mines again, and go out through the exit, which will then take you to the Valley of the Tomb.
(as this exit will collapse when you have used it).
If I can remember this correctly, I think this is
the mine you can flood ? (Put you have to talk someone first, who gives an item to be into a certain place which means that the dwarfs can get out?? Apparently, this happens later in the game].
There is nothing wrong with picking Xan up to be used in your party, and then dismiss him along the way. The game won't be punishing you for this.
To lowert the difficulty in the game, please click on your BG1 icon on your screen, then you should see an options menu. Click on the options menu.
Then there should be a slider which determines the difficulty of the game. Please move the slider all the way to the left. This will make the fihgts a little bit easier, (or a lot).
I can't remember is this also means that there won't be as much XP in rewards when you defeat the enimies or monsters in the game.