1.) Sell the items to a store and buy them back.
2.) From the BGII FAQ on this site:
Q: What is backstab and how do i use it?
A: Backstabbing is an ability available to thieves(except for the swashbuckler kit) and to the ranger kit Stalker. Whenever you make a successful attack on an opponent's back with a piercing weapon (i.e. daggers, short swords, katanas, etc.) whilst being invisible or hiding inshadows, your backstab ability is automatically checked. It multiplies the damage done according to the backstab-multiplier of the character. You can check it in the character screen.
3. You can kill him and take his body to the sphere, but this is evil, although I'm not sure you suffer a reputation loss.
4.) Personally I think Nalia is crap. I think you should pick up Jan Jansen from the Government area, he's a thief/illusionist and he's very good. That also gives you a thief in the party, one is very usefull but enough, so you can boot whoever else you had in mind for a thief.

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)

Nalia might be a slightly better mage, but as a thief she's worthless, hell she can't even pick all the locks in her own castly, let alone the ones you find later in the game.
But since you've got Jan killed, she may be your only option.
Alternatively you could keep Joshi.

BUY them once you've sold items to get them recharged??? steal them back goddammit!!

your party is to melee heavy imo, i usually go with 2 fighters, at least 2 mages/dual mages, a thief and a cleric. much more balanced.

but yes, jan is gone. no way to get him back now that i know of.

nalia isn't all that bad, she's ok as a stop gap.

2 fighters is all the tank power you need to kick everybody's butt.
I normally play with 4 characters, so they get really high level.

EDIT: Untill you get Imoen back off course, after all you're doing all those quests to get her back.
Imoen is a slightly better thief, you can use her very well untill ToB, but then there are new options.

idd, as arathorn said, until you get imoen back.

perhaps a spoiler warning would be nice for the newbie guys....

don't wanna be ruining the plot now would you...

I thought about that, but I don't consider getting Imoen back as a spoiler, because that's really obvious.