Imoen or Valygnar and some other questions

Hi !!! first thanks to everybody that answered my previews message !!! Thanks a Lot from this poor Warrior !!

Now I have some new doubts (I haven't finished the gaem yet so is ma first time...)

1)Now I know i must lose Yoshimo (sniff, he was a great character) and I dismiss him from the party, so I take Valygnar
Now I have a new Questions: There will be problems in the game if i don't taike imoen in the party ? I will miss
some quests or some parts of the plot ?

2)I read in a walkthrough that if i finish the xzar quest and return montaron to him, then i'll must kill the arpist,
if i do so, jahera will leave the party ?

3)I really need a ring of sorcery, where i can find it ? (I need gauntlet of dexterity too, if you know where they
are...)

4)I finish the eyeless cult quest by killing everyone without enjoy the order, so i can't access the secret part of the sewer dungeon... so I use the cheat that show all the map and then the one that transport the party where the mouse cursor is, in this way i found some curious things:

A) How can I get in the Dungeon with the rooms full of mind flyers ? it is a part of which quest ? (now i'm on chapter three so if this quest begin in a future chapter please don't spoil anything

B) How could I get the dongeon with the ghoul's city ? There must be a location from where you can reach the city
but i don't have any idea about its position... if anyone know it....

please be patient with me because is the first time i play this great game (I finish BG1 six times before leave it in the box !)

byez !!!