1) i think you can sell and rebuy them to charge them. But that`s more a trick i guess. Never done that myself, it feels like cheating imo.
2) Dunno if it`s in the FAQ on the website. Basically you must be hidden (invisible or hidden in shadows), then you sneak up right BEHIND an enemy and then you need to hit him. Not all weapons are usable for backstab though. Guess that`s in the FAQ for certain.
3) You have 2 options: Take him with you OR kill him and drag his body to the sphere. So you don`t have much choice. What you CAN do is accept his offer to join you, send Minsc to the Copper Coronet. Go to the planar sphere, open the door, dump Valygar and go pick up Minsc again. I think that`s the easiest way for you. Killing him is not really an option since you seem to have an all good party.
4) One magic user is not much, probably too little. Nalia is decent, although she only has 17 int. You could try and see if you can manage with only Aerie and move on in the game. If it doesn`t work out search for replacement. Nalia is in the copper coronet btw. If you already talked to her she`s at the castle (D`arnise keep).
What`s you current party now btw??
*edit* damn you arathorn
and nalia is not crap, she`s a better mage than Jan (int 17 vs int 16, dualclass vs multiclass)